Fortran: prevent NULL pointer dereferences checking do-loop contained stuff

gcc/fortran/ChangeLog:

	PR fortran/103718
	PR fortran/103719
	* frontend-passes.c (doloop_contained_procedure_code): Add several
	checks to prevent NULL pointer dereferences on valid and invalid
	code called within do-loops.

gcc/testsuite/ChangeLog:

	PR fortran/103718
	PR fortran/103719
	* gfortran.dg/do_check_18.f90: New test.