Fix UB in int_const_binop

Fix UB in int_const_binop

When testing PR 85164, the baseline bootstrap-ubsan results had
a lot of failures from int_const_binop.  This is because with the
new overflow handling we can sometimes do:

      poly_res = res;

on an uninitialised res.

2019-04-18  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* fold-const.c (int_const_binop): Return early on failure.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@270443 138bc75d-0d04-0410-961f-82ee72b054a4