convStrDup() returns NULL if character conversion fails,
so make sure we check the return values and drop the
tag if we can't get a conversion.

This should close bug 1313:
http://musicpd.org/mantis/view.php?id=1313

git-svn-id: https://svn.musicpd.org/mpd/trunk@4641 09075e82-0dd4-0310-85a5-a0d7c8717e4f