Fix fromJust exception in Volume plugin

Suppose there is no `PCM` alsa device.

Xmobar failed with `error: Maybe.fromJust: Nothing` exception running with following config:

    Config { persistent = True
           , lowerOnStart = False
           , hideOnStart = False
           , font = "-*-terminus-medium-r-normal--12-*"
           , border = NoBorder
           , borderColor = "#1c1c1c"
           , bgColor = "#1c1c1c"
           , fgColor = "#8080A1"
           , position = Static { xpos = 120
                               , ypos = 1060
                               , width = 1800
                               , height = 20
                               }
           , commands = [ Run Volume "default" "PCM" [] 5, Run StdinReader ]
           , sepChar = "%"
           , alignSep = "}{"
           , template = " %StdinReader% }{ %default:PCM% "
           }