ALSA: usb-audio: us144mkii: Fix null-deref in tascam_midi_in_urb_complete()

The smatch tool reported a potential null pointer dereference in
tascam_midi_in_urb_complete(). The 'tascam' variable, derived from
'urb->context', was checked for nullity in one place, but dereferenced
without a check in several other places.

This patch fixes the issue by adding a null check at the beginning of
the function. If 'tascam' is null, the function now safely exits.
This prevents any potential crashes from null pointer dereferences.

It also fixes a latent bug where 'usb_put_urb()' could
be called twice for the same URB on submission failure, which would
lead to a use-after-free error.

Fixes: 67afec157fe6 ("ALSA: usb-audio: us144mkii: Add MIDI support and mixer controls")
Reported-by: Dan Carpenter <[e-mail]>
Closes: https://lore.kernel.org/r/[e-mail]/
Signed-off-by: Šerif Rami <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Takashi Iwai <[e-mail]>

sound/usb/usx2y/us144mkii_midi.c

···
 	struct tascam_card *tascam = urb->context;
 	int ret;
 
+	if (!tascam)
+		goto out;
+
 	if (urb->status) {
 		if (urb->status != -ENOENT && urb->status != -ECONNRESET &&
 		    urb->status != -ESHUTDOWN && urb->status != -EPROTO) {
···
 		goto out;
 	}
 
-	if (tascam && atomic_read(&tascam->midi_in_active) &&
+	if (atomic_read(&tascam->midi_in_active) &&
 	    urb->actual_length > 0) {
 		kfifo_in_spinlocked(&tascam->midi_in_fifo, urb->transfer_buffer,
 				    urb->actual_length, &tascam->midi_in_lock);
···
 		dev_err(tascam->card->dev,
 			"Failed to resubmit MIDI IN URB: error %d\n", ret);
 		usb_unanchor_urb(urb);
-		usb_put_urb(urb);
+		goto out;
 	}
+
 out:
 	usb_put_urb(urb);
 }