Fix ICE due to c_safe_arg_type_equiv_p not checking for error_mark node

Fix ICE due to c_safe_arg_type_equiv_p not checking for error_mark node

This is a simple error recovery issue when c_safe_arg_type_equiv_p
was added in r8-5312-gc65e18d3331aa999. The issue is that after
an error, an argument type (of a function type) might turn
into an error mark node and c_safe_arg_type_equiv_p was not ready
for that. So this just adds a check for error operand for its
arguments before getting the main variant.

OK? Bootstrapped and tested on x86_64-linux-gnu with no regressions.

	PR c/101285

gcc/c/ChangeLog:

	* c-typeck.cc (c_safe_arg_type_equiv_p): Return true for error
	operands early.

gcc/testsuite/ChangeLog:

	* gcc.dg/pr101285-1.c: New test.