cdc-acm: fix NULL pointer reference

The union descriptor must be checked. Its usage was conditional
before the parser was introduced. This is important, because
many RNDIS device, which also use the common parser, have
bogus extra descriptors.

Signed-off-by: Oliver Neukum <oneukum@suse.com>
Tested-by: Vasily Galkin <galkin-vv@yandex.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>