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

In the Linux kernel, the following vulnerability has been resolved: 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.