ALSA: usb-audio: Drop snd_BUG_ON() from snd_usbmidi_output_open()

snd_usbmidi_output_open() has a check of the NULL port with
snd_BUG_ON().  snd_BUG_ON() was used as this shouldn't have happened,
but in reality, the NULL port may be seen when the device gives an
invalid endpoint setup at the descriptor, hence the driver skips the
allocation.  That is, the check itself is valid and snd_BUG_ON()
should be dropped from there.  Otherwise it's confusing as if it were
a real bug, as recently syzbot stumbled on it.

Reported-by:  <syzbot+9abda841d636d86c41da@syzkaller.appspotmail.com>
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/syzbot+9abda841d636d86c41da@syzkaller.appspotmail.com
Link: https://lore.kernel.org/r/20221112141223.6144-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>