Language::convertPlural: check if matching form exists

It is possible that only explicit plural forms are specified, and
therefore, it is possible that none match. However, handling of
explicit forms came after the count( $forms ) check, so input such
as {{PLURAL:|1=}} would trigger a "PHP Notice: Undefined offset: -1".

Change-Id: I8494de8ceb9e0cfff7203c69c21f02b3731275af
Follows-Up: I50eb0c6d1c02ca936848d310de625ed1fe43d91a
Merge "Language::convertPlural: check if matching form exists"