Check for the namespace correctly in Special:Allmessages

This is a followup to I21a6d4359faf9d86ce3b01e423e8c3ab4c02658f

It fixes an error there, which causes a lot of messages
not to appear.

This checks for the namespace of the page correctly.

Change-Id: I63a34a4d674454edf9627a5893ef9dd15643f927
(cherry picked from commit 5b67f0effadfb70a2d121c83323db134866ab6ef)