Use correct variable order in add_query_arg(). This had mostly just filled error logs; it also broke some obscure URL situations.

Merges [24444] to the 3.5 branch.

fixes #23284.



git-svn-id: https://core.svn.wordpress.org/branches/3.5@24445 1a063a9b-81f0-0310-95a4-ce76da25c4cd