Keg#unlink: skip find for non-extant files

In 1.8, Pathname.find would work (and do nothing) if the
source path didn't exist. In 1.9 this throws an exception
instead, so check whether the file exists before trying.

Library/Homebrew/keg.rb

@@ -30,6 +30,7 @@ class Keg < Pathname
   def unlink
     n=0
     %w[bin etc lib include sbin share var].map{ |d| self/d }.each do |src|
+      next unless src.exist?
       src.find do |src|
         next if src == self
         dst=HOMEBREW_PREFIX+src.relative_path_from(self)