cleanup: use early return rather than method-spanning conditional

Library/Homebrew/cmd/cleanup.rb

@@ -14,7 +14,8 @@ module Homebrew extend self
   end
 
   def cleanup_all
-    if HOMEBREW_CELLAR.directory?
+    return unless HOMEBREW_CELLAR.directory?
+
     HOMEBREW_CELLAR.children.each do |rack|
       begin
         cleanup_formula Formula.factory(rack.basename.to_s) if rack.directory?
@@ -22,7 +23,6 @@ module Homebrew extend self
         # Don't complain about directories from DIY installs
       end
     end
-    end
     clean_cache
     # seems like a good time to do some additional cleanup
     unless ARGV.dry_run?