c/98200 - improve error recovery for GIMPLE FE

This avoids ICEing by making sure to propagate error early.

2020-12-09  Richard Biener  <rguenther@suse.de>

	PR c/98200
gcc/c/
	* gimple-parser.c (c_parser_gimple_postfix_expression): Return
	early on error.

gcc/testsuite/
	* gcc.dg/gimplefe-error-8.c: New testcase.