Fortran - ICE in gfc_check_do_variable, at fortran/parse.c:4446

Fortran - ICE in gfc_check_do_variable, at fortran/parse.c:4446

Avoid NULL pointer dereferences during error recovery.

gcc/fortran/ChangeLog:

	PR fortran/95501
	PR fortran/95502
	* expr.c (gfc_check_pointer_assign): Avoid NULL pointer
	dereference.
	* match.c (gfc_match_pointer_assignment): Likewise.
	* parse.c (gfc_check_do_variable): Avoid comparison with NULL
	symtree.

gcc/testsuite/ChangeLog:

	PR fortran/95501
	PR fortran/95502
	* gfortran.dg/pr95502.f90: New test.