Fix usage of unset variables (toward fixing #354)
Fix usage of unset variables (toward fixing #354)

set -o nounset
I've only fixed usage of variable that were used at runtime in my
environment. 'ret' remains unfixed.

pkkolos/liquidprompt,alexprengere/liquidprompt,BobHood/liquidprompt,nojhan/liquidprompt,joshuarubin/dotfiles,smadhueagle/liquidprompt,joshuarubin/dotfiles,joshuarubin/dotfiles,jaesivsm/liquidprompt,dan-cleinmark/liquidprompt,joshuarubin/dotfiles,dtschan/liquidprompt,tsbarnes/liquidprompt,dpilone/liquidprompt,laurentbardin/liquidprompt,milouse/liquidprompt