install: f.oldname can be nil

Library/Homebrew/cmd/install.rb

@@ -72,7 +72,7 @@ module Homebrew
         if f.installed?
           msg = "#{f.full_name}-#{f.installed_version} already installed"
           unless f.linked_keg.symlink?
-            if Pathname.new("#{HOMEBREW_CELLAR}/#{f.oldname}").exist?
+            if f.oldname && (HOMEBREW_CELLAR/f.oldname).exist?
               msg << ", it's just not migrated"
             elsif !f.keg_only?
               msg << ", it's just not linked"