eo base - fix free of all callbacks to set callbacks to NULL

we set count to 0 but not ptr to NULL. this we must do after free so
when other destructors run dont go relying on this ptr to be valid!

src/lib/eo/eo_base_class.c

@@ -945,6 +945,7 @@ _eo_callback_remove_all(Efl_Object_Data *pd)
      _eo_callback_free(pd->callbacks[i]);
 
    free(pd->callbacks);
+   pd->callbacks = NULL;
    pd->callbacks_count = 0;
 }