re PR fortran/32594 (substring simplification leads to ICE)

re PR fortran/32594 (substring simplification leads to ICE)

	PR fortran/32594

	* trans-expr.c (gfc_conv_substring_expr): Only call
	gfc_conv_substring if expr->ref is not NULL.
	* expr.c (gfc_is_constant_expr): If e->ref is NULL, the substring
	expression might be a constant.
	(gfc_simplify_expr): Handle missing start and end, as well as
	missing ref.

	* gfortran.dg/substr_5.f90: New test.

From-SVN: r127478