ALSA: usb-audio: Validate MS endpoint descriptors

snd_usbmidi_get_ms_info() may access beyond the border when a
malformed descriptor is passed.  This patch adds the sanity checks of
the given MS endpoint descriptors, and skips invalid ones.

Reported-by:  <syzbot+6bb23a5d5548b93c94aa@syzkaller.appspotmail.com>
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20210510150659.17710-1-tiwai@suse.de


Signed-off-by: Takashi Iwai <tiwai@suse.de>