fold: Change comparison of error_mark_node to use error_operand_p in tree_expr_nonneg...

fold: Change comparison of error_mark_node to use error_operand_p in tree_expr_nonnegative_warnv_p [PR118948]

This is an obvious fix for this small regression. Basically after r15-328-g5726de79e2154a,
there is a call to tree_expr_nonnegative_warnv_p where the type of the expression is now
error_mark_node. Though there was only a check if the expression was error_mark_node.

Bootstrapped and tested on x86_64-linux-gnu.

	PR c/118948

gcc/ChangeLog:

	* fold-const.cc (tree_expr_nonnegative_warnv_p): Use
	error_operand_p instead of checking for error_mark_node directly.

gcc/testsuite/ChangeLog:

	* gcc.dg/pr118948-1.c: New test.

Signed-off-by: Andrew Pinski <quic_apinski@quicinc.com>