PR fortran/31243

	* resolve.c (resolve_substring): Don't allow too large substring
	indexes.
	(gfc_resolve_substring_charlen): Fix typo.
	(gfc_resolve_character_operator): Fix typo.
	(resolve_charlen): Catch unreasonably large string lengths.
	* simplify.c (gfc_simplify_len): Don't error out on LEN
	range checks.

	* gcc/testsuite/gfortran.dg/string_1.f90: New test.
	* gcc/testsuite/gfortran.dg/string_2.f90: New test.
	* gcc/testsuite/gfortran.dg/string_3.f90: New test.


git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@147619 138bc75d-0d04-0410-961f-82ee72b054a4