_handleGeolocationResponse: exception when map is already removed

See #2001

Seeing this error at 

[./src/dom/DomUtil.js:247](https://github.com/Leaflet/Leaflet/blob/0f904a515879fcd08f69b7f51799ee7f18f23fd8/src/dom/DomUtil.js#L247):	`return el._leaflet_pos || new Point(0, 0);`
