inspector: Avoid criticals in the css editro

We can't keep text iters around after text buffer has been
changed. Just clear the list errors when the buffer changes
before we've passed it again.

gtk/inspector/css-editor.c

@@ -308,6 +308,9 @@ text_changed (GtkTextBuffer         *buffer,
     g_source_remove (ce->priv->timeout);
 
   ce->priv->timeout = g_timeout_add (100, update_timeout, ce); 
+
+  g_list_free_full (ce->priv->errors, css_error_free);
+  ce->priv->errors = NULL;
 }
 
 static void