Keg#unlink: Don't try to unlink if keg is unlinked

Library/Homebrew/keg.rb

@@ -29,6 +29,8 @@ class Keg < Pathname
 
   def unlink
     n=0
+    return n if !linked?
+
     %w[bin etc lib include sbin share var].map{ |d| self/d }.each do |src|
       next unless src.exist?
       src.find do |src|