Give a hint, when [] is incorrectly used for a composite type in array.

That used to be accepted, so let's try to give a hint to users on why
their PL/python functions no longer work.

Reviewed by Pavel Stehule.

Discussion: <CAH38_tmbqwaUyKs9yagyRra=SMaT45FPBxk1pmTYcM0TyXGG7Q@mail.gmail.com>

src/pl/plpython/plpy_cursorobject.c

@@ -240,7 +240,8 @@ PLy_cursor_plan(PyObject *ob, PyObject *args)
 					plan->values[j] =
 						plan->args[j].out.d.func(&(plan->args[j].out.d),
 												 -1,
-												 elem);
+												 elem,
+												 false);
 				}
 				PG_CATCH();
 				{