staging: nvec: potential NULL dereference on error path

We assume nvec->rx can be NULL earlier so I have added a check here as
well.
Signed-off-by: NDan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/nvec/nvec.c

@@ -681,7 +681,8 @@ static irqreturn_t nvec_interrupt(int irq, void *dev)
 			dev_err(nvec->dev,
 				"RX buffer overflow on %p: "
 				"Trying to write byte %u of %u\n",
-				nvec->rx, nvec->rx->pos, NVEC_MSG_SIZE);
+				nvec->rx, nvec->rx ? nvec->rx->pos : 0,
+				NVEC_MSG_SIZE);
 		break;
 	default:
 		nvec->state = 0;