restore_current_blog() in add_user_to_blog() and remove_user_from_blog()...

restore_current_blog() in add_user_to_blog() and remove_user_from_blog() before early return, to avoid being stuck in a switched state. props mdawaffe. fixes #16444 for trunk.

git-svn-id: https://core.svn.wordpress.org/trunk@17379 1a063a9b-81f0-0310-95a4-ce76da25c4cd