Fix infinite loop for self-redirects with variants conversion
The redirect targets another variant, but the target does not exist.
It causes infinite loop when querying the page with the redirect
and converttitles flags are both set.
Check the mConvertTitles to stop re-resolving the same title.
Bug: T333050
Change-Id: I64c5886dbd0a9148eeff033ba56306bcb7f2eb8c
(cherry picked from commit 24c3ef2474c6daa20ed48168d46196a55346dfd8)