Fix fatal error in Special:SetSiteLink

Bug: 63003
Change-Id: I9cec5c4507fdccb7e6baf3bc806e0dae955442a9
(cherry picked from commit 1e11c570c37e47e820eeff8608d5ccd13711ac09)
Merge "Fix fatal error in Special:SetSiteLink" into mw1.23-wmf19