PR fortran/93423 - ICE on invalid with argument list for module procedure

When recovering from an error, a NULL pointer dereference could occur.
Check for that situation and punt.

gcc/fortran/
	PR fortran/93423
	* resolve.c (resolve_symbol): Avoid NULL pointer dereference.

gcc/fortran/resolve.c

@@ -15918,7 +15918,7 @@ resolve_symbol (gfc_symbol *sym)
       if (formal)
 	{
 	  sym->formal_ns = formal->sym->ns;
-          if (sym->ns != formal->sym->ns)
+	  if (sym->formal_ns && sym->ns != formal->sym->ns)
 	    sym->formal_ns->refs++;
 	}
     }

gcc/testsuite/gfortran.dg/pr93423.f90

@@ -0,0 +1,21 @@
+! { dg-do compile }
+! PR fortran/93423 - ICE on invalid with argument list for module procedure
+
+module t
+  type :: b
+   contains
+     procedure :: p => bp
+  end type b
+  interface
+     module function bp(s)
+       class(b), intent(inout) :: s
+       integer, pointer :: bp
+     end function
+  end interface
+end module t
+
+submodule (t) ts
+contains
+  module procedure bp(s) ! { dg-error "must be in a generic module interface" }
+  end procedure bp       ! { dg-error "Expecting END SUBMODULE statement" }
+end submodule ts