mapbox: fix loc.context.length undefined
mapbox: fix loc.context.length undefined

Fixes #254.

perliedman/leaflet-control-geocoder,wbg/leaflet-control-geocoder,perliedman/leaflet-control-geocoder,wbg/leaflet-control-geocoder,perliedman/leaflet-control-geocoder,wbg/leaflet-control-geocoder