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>

drivers/staging/vt6656/rxtx.c

@@ -148,6 +148,8 @@ static void *s_vGetFreeContext(struct vnt_private *pDevice)
     DBG_PRT(MSG_LEVEL_DEBUG, KERN_INFO"GetFreeContext()\n");
 
     for (ii = 0; ii < pDevice->cbTD; ii++) {
+	if (!pDevice->apTD[ii])
+		return NULL;
         pContext = pDevice->apTD[ii];
         if (pContext->bBoolInUse == false) {
             pContext->bBoolInUse = true;