Fortran: prevent NULL pointer dereference in check of passed do-loop variable

gcc/fortran/ChangeLog:

	PR fortran/103717
	* frontend-passes.c (doloop_code): Prevent NULL pointer
	dereference when checking for passing a do-loop variable to a
	contained procedure with an interface mismatch.

gcc/testsuite/ChangeLog:

	PR fortran/103717
	* gfortran.dg/do_check_19.f90: New test.