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.

liquidprompt

@@ -1065,7 +1065,7 @@ _lp_hostname_color() {
             ;;
         esac
     else
-        if [[ -n $lp_chroot ]]; then
+        if [[ -n ${lp_chroot-} ]]; then
             # End the color of the chroot
             lp_hostname_color+=${NO_COL}
         else