c/100625 - avoid building invalid labels in the GIMPLE FE

When duplicate labes are diagnosed, avoid building a GIMPLE_LABEL.

2021-05-17  Richard Biener  <rguenther@suse.de>

	PR c/100625
gcc/c/
	* gimple-parser.c (c_parser_gimple_label): Avoid building
	a GIMPLE label with NULL label decl.

gcc/testsuite/
	* gcc.dg/gimplefe-error-9.c: New testcase.