[decoder] fix for buf=NULL, size=0 case. (#246)

tests/test_generated_code.c

@@ -340,11 +340,23 @@ void test_repeated() {
   upb_arena_free(arena);
 }
 
+void test_null_decode_buf() {
+  upb_arena *arena = upb_arena_new();
+  protobuf_test_messages_proto3_TestAllTypesProto3 *msg =
+      protobuf_test_messages_proto3_TestAllTypesProto3_parse(NULL, 0, arena);
+  size_t size;
+
+  ASSERT(msg);
+  protobuf_test_messages_proto3_TestAllTypesProto3_serialize(msg, arena, &size);
+  ASSERT(size == 0);
+}
+
 int run_tests(int argc, char *argv[]) {
   test_scalars();
   test_string_map();
   test_string_double_map();
   test_int32_map();
   test_repeated();
+  test_null_decode_buf();
   return 0;
 }

upb/decode.c

@@ -622,6 +622,9 @@ bool upb_decode(const char *buf, size_t size, void *msg, const upb_msglayout *l,
   state.depth = 64;
   state.end_group = 0;
 
+  /* Early exit required for buf==NULL case. */
+  if (size == 0) return true;
+
   CHK(upb_decode_message(buf, l, msg, &state));
   return state.end_group == 0;
 }