[media] dvb-usb: check for invalid length in ttusb_process_muxpack()

This patch is driven by a static checker warning.
The ttusb_process_muxpack() function is only called from
ttusb_process_frame().  Before calling, it verifies that len >= 2.  The
problem is that len == 2 is not valid and would lead to an array
underflow.
Odd number values for len are also invalid and would lead to reading
past the end of the array.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>