app: don't crash on a dangling image_map_tool->active_picker pointer

gimp_operation_tool_set_operation(): set it to NULL when destroying
the previous operation's options_table (if it was non-NULL it was a
widget from that table).

app/tools/gimpoperationtool.c

@@ -312,6 +312,8 @@ gimp_operation_tool_set_operation (GimpOperationTool *tool,
     {
       gtk_widget_destroy (tool->options_table);
       tool->options_table = NULL;
+
+      GIMP_IMAGE_MAP_TOOL (tool)->active_picker = NULL;
     }
 
   if (tool->config)