Fix PR fortran/93500, ICE on invalid.

Returning &gfc_bad_expr when simplifying bounds after a divisin by zero
happened results in the division by zero error actually reaching the user.

2020-04-19  Thomas Koenig  <tkoenig@gcc.gnu.org>

	PR fortran/93500
	* resolve.c (resolve_operator): If both operands are
	NULL, return false.
	* simplify.c (simplify_bound): If a division by zero
	was seen during bound simplification, free the
	corresponcing expression and return &gfc_bad_expr.

2020-04-19  Thomas Koenig  <tkoenig@gcc.gnu.org>

	PR fortran/93500
	* arith_divide_3.f90: New test.