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

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

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.