ALSA: usb-audio: Fix potential memory leaks at error path for UMP open

The allocation and initialization errors at alloc_midi_urbs() that is
called at MIDI 2.0 / UMP device are supposed to be handled at the
caller side by invoking free_midi_urbs().  However, free_midi_urbs()
loops only for ep->num_urbs entries, and since ep->num_entries wasn't
updated yet at the allocation / init error in alloc_midi_urbs(), this
entry won't be released.

The intention of free_midi_urbs() is to release the whole elements, so
change the loop size to NUM_URBS to scan over all elements for fixing
the missed releases.

Also, the call of free_midi_urbs() is missing at
snd_usb_midi_v2_open().  Although it'll be released later at
reopen/close or disconnection, it's better to release immediately at
the error path.

Fixes: ff49d1df ("ALSA: usb-audio: USB MIDI 2.0 UMP support")
Reported-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Closes: https://lore.kernel.org/r/fc275ed315b9157952dcf2744ee7bdb78defdb5f.1693746347.git.christophe.jaillet@wanadoo.fr
Link: https://lore.kernel.org/r/20230905054511.20502-1-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/midi2.c

@@ -265,7 +265,7 @@ static void free_midi_urbs(struct snd_usb_midi2_endpoint *ep)
 
 	if (!ep)
 		return;
-	for (i = 0; i < ep->num_urbs; ++i) {
+	for (i = 0; i < NUM_URBS; ++i) {
 		ctx = &ep->urbs[i];
 		if (!ctx->urb)
 			break;
@@ -279,6 +279,7 @@ static void free_midi_urbs(struct snd_usb_midi2_endpoint *ep)
 }
 
 /* allocate URBs for an EP */
+/* the callers should handle allocation errors via free_midi_urbs() */
 static int alloc_midi_urbs(struct snd_usb_midi2_endpoint *ep)
 {
 	struct snd_usb_midi2_urb *ctx;
@@ -351,8 +352,10 @@ static int snd_usb_midi_v2_open(struct snd_ump_endpoint *ump, int dir)
 		return -EIO;
 	if (ep->direction == STR_OUT) {
 		err = alloc_midi_urbs(ep);
-		if (err)
+		if (err) {
+			free_midi_urbs(ep);
 			return err;
+		}
 	}
 	return 0;
 }