internal-fn: Fix up expand_arith_overflow [PR114753]

During backporting I've noticed I've missed one return spot for the
restoration of the original flag_trapv flag value.

2024-04-19  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/114753
	* internal-fn.cc (expand_arith_overflow): Add one missing restore
	of flag_trapv before return.