Fix handling of duplicate album artists in ftintitle

The code that determines the featured artist part of the artist only
handles two element lists. When the album artist is duplicated, it
splits into a three element list causing no featured artist to be found.

This ensures the albumartist_split can ever only have 2 elements.

Fixes #1179
Merge pull request #1181 from marcaddeo/fix-ftintitle-duplicate-album-artist

Fix handling of duplicate album artists in ftintitle
Changelog for #1181