src: Fix possible heap-use-after free for OpenSSL global locking

This is simply programming error, but it is interesting that using
libstdc++ does not reveal this error.  With clang++-libc++, we got
std::system_error: mutex lock faild: Invalid argument.  This is
because we did not give a name to lock object, so it is immediately
destructed.  I think this will fix the reported crash on Mac OSX.

src/h2load.cc

@@ -812,7 +812,7 @@ int main(int argc, char **argv)
   SSL_library_init();
 
 #ifndef NOTHREADS
-  ssl::LibsslGlobalLock();
+  ssl::LibsslGlobalLock lock;
 #endif // NOTHREADS
 
   auto ssl_ctx = SSL_CTX_new(SSLv23_client_method());

src/nghttpd.cc

@@ -296,7 +296,7 @@ int main(int argc, char **argv)
   SSL_load_error_strings();
   SSL_library_init();
 #ifndef NOTHREADS
-  ssl::LibsslGlobalLock();
+  ssl::LibsslGlobalLock lock;
 #endif // NOTHREADS
 
   reset_timer();

src/shrpx.cc

@@ -1621,7 +1621,7 @@ int main(int argc, char **argv)
   SSL_load_error_strings();
   SSL_library_init();
 #ifndef NOTHREADS
-  nghttp2::ssl::LibsslGlobalLock();
+  nghttp2::ssl::LibsslGlobalLock lock;
 #endif // NOTHREADS
 
   if(conf_exists(get_config()->conf_path.get())) {