filechooser: Only do search_start_query() while we are in search mode

This signal can be emitted by GtkSearchEntry after search has been
cancelled, and other operation mode is set. It doesn't make sense to
populate the search model in that state anymore, so just avoid doing it.

https://bugzilla.gnome.org/show_bug.cgi?id=745479

gtk/gtkfilechooserwidget.c

@@ -6304,6 +6304,9 @@ search_entry_activate_cb (GtkFileChooserWidget *impl)
   GtkFileChooserWidgetPrivate *priv = impl->priv;
   const char *text;
 
+  if (priv->operation_mode != OPERATION_MODE_SEARCH)
+    return;
+
   text = gtk_entry_get_text (GTK_ENTRY (priv->search_entry));
 
   /* reset any existing query object */