Fix unset error in _lp_hostname_color()
Fix unset error in _lp_hostname_color()

I added this one recently. It is quite hard to keep up with these unset
variable errors in a language like this, especially when I can't run set
-u all the time myself because of autocomplete not supporting it.

nojhan/liquidprompt,milouse/liquidprompt,jaesivsm/liquidprompt,laurentbardin/liquidprompt,pkkolos/liquidprompt