adding extra safety check in BaseZoomTool
adding extra safety check in BaseZoomTool

ContinuumIO/chaco,burnpanck/chaco,tommy-u/chaco,burnpanck/chaco,ContinuumIO/chaco,tommy-u/chaco,burnpanck/chaco,ContinuumIO/chaco,tommy-u/chaco,ContinuumIO/chaco