ALSA: opl3: Fix possible negative array index access

Spotted by coverity CID 115196.

Signed-off-by: Takashi Iwai <[e-mail]>

sound/drivers/opl3/opl3_midi.c

···
 		voice = snd_opl3_oss_map[chan->number];		
 	}
 
+	if (voice < 0) {
+		spin_unlock_irqrestore(&opl3->voice_lock, flags);
+		return;
+	}
+
 	if (voice < MAX_OPL2_VOICES) {
 		/* Left register block for voices 0 .. 8 */
 		reg_side = OPL3_LEFT;