s390/3270: Fix buffer assignment

Since commit 1b2ac5a6 ("s390/3270: use new address translation
helpers") rq->buffer is passed unconditionally to virt_to_dma32().
The 3270 driver allocates requests without buffer, so the value passed
to virt_to_dma32 might be NULL. Check for NULL before assigning.

Fixes: 1b2ac5a6 ("s390/3270: use new address translation helpers")
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>

drivers/s390/char/raw3270.c

@@ -162,7 +162,8 @@ struct raw3270_request *raw3270_request_alloc(size_t size)
 	/*
 	 * Setup ccw.
 	 */
-	rq->ccw.cda = virt_to_dma32(rq->buffer);
+	if (rq->buffer)
+		rq->ccw.cda = virt_to_dma32(rq->buffer);
 	rq->ccw.flags = CCW_FLAG_SLI;
 
 	return rq;
@@ -188,7 +189,8 @@ int raw3270_request_reset(struct raw3270_request *rq)
 		return -EBUSY;
 	rq->ccw.cmd_code = 0;
 	rq->ccw.count = 0;
-	rq->ccw.cda = virt_to_dma32(rq->buffer);
+	if (rq->buffer)
+		rq->ccw.cda = virt_to_dma32(rq->buffer);
 	rq->ccw.flags = CCW_FLAG_SLI;
 	rq->rescnt = 0;
 	rq->rc = 0;