Don't raise when expanding ORIGINAL_PATHS

File.expand_path raises ArgumentError when it is passed a path with a
nonexistent username, e.g.:

  $ PATH=~foo/bin:$PATH brew
  /usr/local/Library/Homebrew/global.rb:97:in `expand_path': user foo
  doesn't exist (ArgumentError)

However, `brew doctor` does its own expansion of PATH entries and
outputs warnings if this happens, so let's just ignore it here and
continue on our way.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Library/Homebrew/global.rb

@@ -94,4 +94,4 @@ unless ARGV.include? "--no-compat" or ENV['HOMEBREW_NO_COMPAT']
   require 'compatibility'
 end
 
-ORIGINAL_PATHS = ENV['PATH'].split(':').map{ |p| Pathname.new(File.expand_path(p)) }
+ORIGINAL_PATHS = ENV['PATH'].split(':').map{ |p| Pathname.new(File.expand_path(p)) rescue nil }.compact.freeze