libkb: don't panic if there's no user when getting mount dir
libkb: don't panic if there's no user when getting mount dir

This can apparently legitimately happen in the iOS simulator.

Issue: HOTPOT-1041

keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client