gir-parser: fix warnings for non set symbol
This work patch a set of warnings used to be thrown when parsing a GIR file to generate a VAPI. Lot of messages like:
** (vapigen:6843): CRITICAL **: 12:42:46.335: vala_gir_parser_is_container: assertion 'sym != NULL' failed
** (vapigen:6843): CRITICAL **: 12:42:46.335: vala_gir_parser_is_container: assertion 'sym != NULL' failed
** (vapigen:6843): CRITICAL **: 12:42:46.395: vala_gir_parser_add_symbol_to_container: assertion 'sym != NULL' failed
now has gone.