libopenmpt: add missing avio_read return value check

This fixes heap-buffer-overflows in libopenmpt caused by interpreting
the negative size value as unsigned size_t.

Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
Reviewed-by: Jörn Heusipp <osmanx@problemloesungsmaschine.de>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
