Masquerade causing edit button to appear on user/UID/edit/insertotherextrapath

See http://gyazo.com/793a1e8a1ee8f8fd944d58a9784fc003.png

It's two bugs, only one of which is Masquerade's:

1) User module isn't verifying a category exists
2) Masquerade doesn't check category when form altering

Sounds similar to #693582: Masquerade options appearing in other tabs of user edit screen but that's long fixed.