(bug 71109) Fix broken Special:EditWatchlist

On enwiki, there was some invalid value in watchlist table.
2 people are watching some title called "The_Washington_&_Jefferson_College_Review"
in namespace 2600 (topic namespace)

That namespace should only have topic ids. On that assumption, we didn't
properly check if that is in fact the case, and created a UUID object
from an incorrect value, causing an exception.

Bug: 71109
Change-Id: I211559979af9b900dbcecb82ac2f4c9bf691f6f8