Fortran: Fix ICE due to referencing a NULL pointer [PR100018]

gcc/fortran/ChangeLog:

	PR fortran/100018
	* resolve.c: Add association check before de-referencing pointer.

gcc/testsuite/ChangeLog:

	PR fortran/100018
	* gfortran.dg/PR10018.f90: New test.

gcc/fortran/resolve.c

@@ -11999,6 +11999,7 @@ start:
 	    /* Assigning a class object always is a regular assign.  */
 	    if (code->expr2->ts.type == BT_CLASS
 		&& code->expr1->ts.type == BT_CLASS
+		&& CLASS_DATA (code->expr2)
 		&& !CLASS_DATA (code->expr2)->attr.dimension
 		&& !(gfc_expr_attr (code->expr1).proc_pointer
 		     && code->expr2->expr_type == EXPR_VARIABLE

gcc/testsuite/gfortran.dg/PR10018.f90

@@ -0,0 +1,11 @@
+! { dg-do compile }
+!
+subroutine foo(that)
+  implicit none
+  class(*),  target, intent(in)  :: this
+  class(*), pointer, intent(out) :: that
+
+  that => this
+  return
+end subroutine foo
+! { dg-error "Symbol at \\\(1\\\) is not a DUMMY variable" "" { target "*-*-*" } 5 }