ALSA: usx2y: us144mkii: fix NULL deref on missing interface 0

A malicious USB device with the TASCAM US-144MKII device id can have a
configuration containing bInterfaceNumber=1 but no interface 0.  USB
configuration descriptors are not required to assign interface numbers
sequentially, so usb_ifnum_to_if(dev, 0) returns will NULL, which will
then be dereferenced directly.

Fix this up by checking the return value properly.

Cc: Jaroslav Kysela <perex@perex.cz>
Cc: Takashi Iwai <tiwai@suse.com>
Fixes: dee1bcf28a3d ("ALSA: usx2y: Add native ALSA driver for TASCAM US-144MKII")
Cc: stable <stable@kernel.org>
Assisted-by: gregkh_clanker_t1000
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://patch.msgid.link/2026040955-fall-gaining-e338@gregkh
Signed-off-by: Takashi Iwai <tiwai@suse.de>