staging: vt6656: rxtx.c [BUG] s_vGetFreeContext dead lock on null apTD.

There seems to be race condition that the device is ndo_start_xmit
at a point where the device is closing and apTD is NULL resulting
in dead lock.

Add a NULL check to apTD and return NULL to calling functions.

This is more likely on 64 bit systems.

Signed-off-by: Malcolm Priestley <tvboxspy@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>