Fix invalid instrument recover in IT event reader

When a note without instrument is used after an invalid instrument is
specified, recover the invalid instrument and ignore the new note. See
the OpenMPT InstrumentNumberChange.it test case.

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
[test] Check module or data file opening errors

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
[test] Add OpenMPT InstrumentNumberChange.it test case

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