Explicitly propagate the status of Flush().

Before the change, an implicit Flush() will be triggered in the
destructor of the input stream. However, the return code of Flush() is
not discarded. This change makes sure when Flush() fails, we will
return false.

src/google/protobuf/message.cc

@@ -170,12 +170,12 @@ size_t Message::SpaceUsedLong() const {
 
 bool Message::SerializeToFileDescriptor(int file_descriptor) const {
   io::FileOutputStream output(file_descriptor);
-  return SerializeToZeroCopyStream(&output);
+  return SerializeToZeroCopyStream(&output) && output.Flush();
 }
 
 bool Message::SerializePartialToFileDescriptor(int file_descriptor) const {
   io::FileOutputStream output(file_descriptor);
-  return SerializePartialToZeroCopyStream(&output);
+  return SerializePartialToZeroCopyStream(&output) && output.Flush();
 }
 
 bool Message::SerializeToOstream(std::ostream* output) const {