wwaninfo: don't free signals when not connected

Another issue found when building with -O0

src/wwaninfo.c

@@ -235,8 +235,10 @@ phosh_wwan_info_dispose (GObject *object)
 
   if (priv->wwan) {
     for (int i=0; i < WWAN_INFO_WWAN_NUM_SIGNALS; i++) {
-      g_signal_handler_disconnect (priv->wwan, priv->wwan_signal_ids[i]);
-      priv->wwan_signal_ids[i] = 0;
+      if (priv->wwan_signal_ids[i] > 0) {
+        g_signal_handler_disconnect (priv->wwan, priv->wwan_signal_ids[i]);
+        priv->wwan_signal_ids[i] = 0;
+      }
     }
     g_clear_object (&priv->wwan);
   }