app: get rid of a wrong return-after-free report.

Reported by Massimo, though this one is wrong, as far as I can see.

`ninja scan-build` apparently reports `result` as being returned after
being freed here. But actually since we are setting the `error` with
g_set_error() in the same time we free `result`, we would also enter the
`if (*error)` block a few lines later, which would return NULL and not
`result` anyway. I guess the static analyzer could not see that far.

Still, set the pointer to NULL with g_clear_pointer(), which should be
enough to make the static analyzer happy.

app/propgui/gimppropgui-eval.c

@@ -655,7 +655,7 @@ gimp_prop_eval_string_impl (GObject      *config,
       g_set_error (error, GIMP_PROP_EVAL_ERROR, GIMP_PROP_EVAL_FAILED,
                    "invalid expression");
 
-      g_free (result);
+      g_clear_pointer (&result, g_free);
     }
 
   g_free (t);