Fix stray notes if last instrument is invalid

In FT2, if a note has no instrument and the last specified instrument is
invalid, don't play the note. See last notes of mothershipftw-stroberev2.xm.
Reported by Andreas Argirakis.

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
[test] Add test for FT2 new note after invalid ins

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>