support FOLLY_SAFE_CHECK with no message args

Summary:
Permit calls of the form:

```lang=c++
FOLLY_SAFE_DCHECK(expr);
```

Reviewed By: ot

Differential Revision: D27843903

fbshipit-source-id: 23869720822fcdba0845c42279855b101c8e2b36

folly/lang/SafeAssert.h

@@ -47,7 +47,8 @@
                   ::folly::detail::safe_assert_msg_types_seq_of(        \
                       __VA_ARGS__))>::value.data};                      \
       ::folly::detail::safe_assert_terminate<p>(                        \
-          __folly_detail_safe_assert_arg, __VA_ARGS__);           \
+          __folly_detail_safe_assert_arg FOLLY_PP_DETAIL_APPEND_VA_ARG( \
+              __VA_ARGS__));                                            \
     }                                                                   \
   } while (false)
 

folly/lang/test/SafeAssertTest.cpp

@@ -49,6 +49,16 @@ TEST(SafeAssert, AssertionFailure) {
   EXPECT_DEATH(fail(), "Message: hello");
 }
 
+TEST(SafeAssert, AssertionFailureVariadicFormattedArgs) {
+  FOLLY_SAFE_CHECK(true);
+  EXPECT_DEATH( //
+      ([] { FOLLY_SAFE_CHECK(false); }()), //
+      "Assertion failure: false");
+  EXPECT_DEATH( //
+      ([] { FOLLY_SAFE_CHECK(false, 17ull, " < ", 18ull); }()), //
+      "Message: 17 < 18");
+}
+
 TEST(SafeAssert, AssertionFailureErrno) {
   EXPECT_DEATH(
       ([] { FOLLY_SAFE_PCHECK((errno = EINVAL) && false, "hello"); }()),