protobuf-c.c: Better input checks in protobuf_c_message_free_unpacked

Check for NULL message pointer passed to protobuf_c_message_free_unpacked

Closes #177

protobuf-c/protobuf-c.c

@@ -3158,10 +3158,14 @@ void
 protobuf_c_message_free_unpacked(ProtobufCMessage *message,
 				 ProtobufCAllocator *allocator)
 {
+	if (message == NULL)
+		return;
+
 	const ProtobufCMessageDescriptor *desc = message->descriptor;
 	unsigned f;
 
 	ASSERT_IS_MESSAGE(message);
+
 	if (allocator == NULL)
 		allocator = &protobuf_c__allocator;
 	message->descriptor = NULL;

t/generated-code2/test-generated-code2.c

@@ -1925,6 +1925,12 @@ test_alloc_fail (void)
   free (packed);
 }
 
+static void
+test_free_unpacked_input_check_for_null_message (void)
+{
+  protobuf_c_message_free_unpacked (NULL, NULL);
+}
+
 /* This test checks that protobuf decoder is capable of detecting special
    cases of incomplete messages. The message should have at least two required
    fields field1 and field129 with positions pos1 and pos2 (no matter what the
@@ -2225,6 +2231,8 @@ static Test tests[] =
   { "test free unpacked", test_alloc_free_all },
   { "test alloc failure", test_alloc_fail },
 
+  { "test free unpacked input check for null message", test_free_unpacked_input_check_for_null_message },
+
   { "test required_fields_bitmap", test_required_fields_bitmap },
 
   { "test field flags", test_field_flags },