ALSA: opl3: small array underflow

There is a missing lower bound check on "pitchbend" so it means we can
read up to 6 elements before the start of the opl3_note_table[] array.

Thanks to Clemens Ladisch for his help with this patch.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>