ALSA: usb-audio: Fix potential out-of-bounce access in MIDI EP parser

The recently introduced MIDI endpoint parser code has an access to the
field without the size validation, hence it might lead to
out-of-bounce access.  Add the sanity checks for the descriptor
sizes.

Fixes: eb596e0f ("ALSA: usb-audio: generate midi streaming substream names from jack names")
Link: https://lore.kernel.org/r/20210511090500.2637-1-tiwai@suse.de


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