Check if tag is equal to 0

Fix a SEGV on on protobuf_c_message_check on messages with 'oneof's inside

* allow compilation with -pedantic (thanks @Bun)

Can be reproduced with the following command:

$ make CFLAGS=-pedantic protobuf-c/protobuf-c.o

* put el-size on a separate line

Fix proto3 repeated scalar field default packing behavior (#330)

Fixed out-of-bounds read (#375)

Fixes #330 and makes scalar repeated fields packed by default in proto3.

https://developers.google.com/protocol-buffers/docs/encoding#packed:
In proto3, repeated fields of scalar numeric types are packed by
default.

The scan_length_prefixed_data() function returns the number of bytes
taken up by a varint length delimiter, plus the actual value of that
delimiter. Since it returns a uint32_t, a delimiter of 2^32 - 1 (or
close to that) could cause the return value to overflow and result in an
incorrect value.

At first I tried to fix it by making scan_length_prefixed_data() use a
size_t for its result, but I realized this would have no effect on
32-bit systems. To fix the problem for 32-bit, I changed the function to
return early if the length is 2 GiB or more (protobuf messages are not
allowed to be that large). I kept the size_t change anyway, since the
result will ultimately be stored in a size_t (ScannedMember.len) and we
might as well stay consistent with that.
Signed-off-by: Adam Cozzette <acozzette@google.com>

Signed-off-by: Hayri Ugur Koltuk <ugur.koltuk@adyen.com>

t/generated-code2/cxx-generate-packed-data.cc: Fix build failure on newer protobuf

google::protobuf::Message::Reflection has been removed in newer versions
of protobuf. The replacement is google::protobuf::Reflection.

protobuf in commit 779f61c6a3ce02a119e28e802f229e61b69b9046 ("Integrate
recent changes from google3.", from August 2008) changed the following
in message.h:

    @@ -336,7 +337,8 @@ class LIBPROTOBUF_EXPORT Message {

       // Introspection ---------------------------------------------------

    -  class Reflection;  // Defined below.
    +  // Typedef for backwards-compatibility.
    +  typedef google::protobuf::Reflection Reflection;

The "typedef for backwards-compatibility" apparently lasted ten years
until protobuf commit 6bbe197e9c1b6fc38cbdc45e3bf83fa7ced792a3
("Down-integrate from google3.", from August 2018) which finally removed
the typedef:

    @@ -327,8 +344,6 @@ class LIBPROTOBUF_EXPORT Message : public MessageLite {

       // Introspection ---------------------------------------------------

    -  // Typedef for backwards-compatibility.
    -  typedef google::protobuf::Reflection Reflection;

This commit updates the only use of this typedef (in the test suite) to
directly refer to the replacement, google::protobuf::Reflection. This
fixes the build failure in the test suite.

.travis.yml: Bump protobuf version to 3.7.1, use new URL

Signed-off-by: Hayri Ugur Koltuk <ugur.koltuk@adyen.com>

Bump version to 1.3.1

Use AX_CXX_COMPILE_STDCXX macro to enable C++11 support in old compilers

This reverts commit 0585e053.

Now that we autodetect any needed flags to enable C++11 support with the
AX_CXX_COMPILE_STDCXX macro, we no longer need to force this in
.travis.yml.

This commit enables use of the AX_CXX_COMPILE_STDCXX macro from the
autoconf-archive (added in the previous commit), which automatically
adds needed flags to the CXX variable to enable C++11 support. This only
works and is only required on compilers that are both new enough to have
C++11 support and old enough to not enable it by default.

We call it with "noext" as the second argument (so that we get the "std"
C++11 variant on GCC) and "mandatory" as the third argument, since we
now rely on C++11 features as of 83c59e70.

Note that this macro is only called when we are building the compiler
(i.e., without --disable-protoc) so that the library can still be built
on ancient C compilers.

fix mingw compilation

Convert uses of protobuf's scoped_ptr.h to C++11 std::unique_ptr

Upstream protobuf removed scoped_ptr.h, which provided the scoped_ptr
that we depended on, in commit 67952fab2c766ac5eacc15bb78e5af4039a3d398
(https://github.com/google/protobuf/commit/67952fab2c766ac5eacc15bb78e5af4039a3d398).

This commit converts uses of scoped_ptr type to idiomatic C++11
std::unique_ptr, similar to the changes in the protobuf commit.

This has been tested on protobuf 3.0.0 (the version currently in Debian)
as well as the latest protobuf 3.6.1 release.

Add std:: to some types

Update Travis to use xenial, protobuf 3.6.1

The xenial Travis-CI environment uses a g++ version that is old enough
that it doesn't use C++11 or newer by default, so we have to force it by
setting CXX to "g++ -std=c++11".

This commit updates the base Travis-CI image from trusty to xenial,
which ships with a slightly more modern compiler.

This is required for compilation to succeed on debian jessie with g++
6.3.0.

This is needed because in the syntax of Google protobuf the zero
value is not valid tag value. The specification
(https://developers.google.com/protocol-buffers/docs/proto3) says
that "The smallest tag number you can specify is 1, and the largest
is 2^29-1, or 536,870,911."

Restore protobuf-2.x compatibility

The change in commit 712154b9 ("Bump
minimum required header version for proto3 syntax") uses functionality
only exposed by protobuf-3.x, breaking the build when compiling against
protobuf-2.x.

Since we still want to support building against protobuf-2.x, this
commit makes the proto3 syntax check in the file generator dependent on
building against protobuf-3.x.

ChangeLog: 1.3.0