cask/postflight: Check QLStephen existence before copying blacklist
cask/postflight: Check QLStephen existence before copying blacklist

Previously I was assuming QLStephen.qlgenerator exists either in
/Library/QuickLook or ~/Library/QuickLook, but that's apparently not a
foolproof assumption. Although nonexistence in either location would
have led to "Copy operation failed", it's still good to confirm the
existence beforehand.

zmwangx/dotfiles,zmwangx/dotfiles,zmwangx/dotfiles,zmwangx/dotfiles,zmwangx/dotfiles