Gracefully handle invalid links when handling redlinks

There is no guarantee just because a link was created by parsoid that it will
resolve to a valid Title object.  If the link doesn't resolve to a valid title
then just ignore it.

Bug: 57904
Change-Id: I85ce508de311afb4ff811a1b6a8a62a7e4c5b4bf
Merge "Gracefully handle invalid links when handling redlinks"