Fix NoMethodError when check_dir fails to enum the permission

See the following for more info:
http://dev.metasploit.com/redmine/issues/7452

Author: sinn3r

modules/post/windows/gather/enum_dirperms.rb

@@ -100,6 +100,7 @@ def enum_subdirs(dpath, maxdepth, token)
 				realpath = dpath + '\\' + d
 				if session.fs.file.stat(realpath).directory?
 					perm = check_dir(realpath, token)
+					next if perm.nil?
 					if !filter or perm.include? filter
 						print_status(perm + "\t" + realpath)
 					end