virtio-ccw: Fix unsetting of indicators.

Interpretation of the ccws to register (configuration) indicators contained
a thinko: We want to disallow reading from 0, but setting the indicator
pointer to 0 is fine.

Let's fix the handling for CCW_CMD_SET{,_CONF}_IND.

Cc: qemu-stable@nongnu.org
Signed-off-by: NCornelia Huck <cornelia.huck@de.ibm.com>

hw/s390x/virtio-ccw.c

@@ -328,10 +328,10 @@ static int virtio_ccw_cb(SubchDev *sch, CCW1 ccw)
             ret = -EINVAL;
             break;
         }
-        indicators = ldq_phys(ccw.cda);
-        if (!indicators) {
+        if (!ccw.cda) {
             ret = -EFAULT;
         } else {
+            indicators = ldq_phys(ccw.cda);
             dev->indicators = indicators;
             sch->curr_status.scsw.count = ccw.count - sizeof(indicators);
             ret = 0;
@@ -348,10 +348,10 @@ static int virtio_ccw_cb(SubchDev *sch, CCW1 ccw)
             ret = -EINVAL;
             break;
         }
-        indicators = ldq_phys(ccw.cda);
-        if (!indicators) {
+        if (!ccw.cda) {
             ret = -EFAULT;
         } else {
+            indicators = ldq_phys(ccw.cda);
             dev->indicators2 = indicators;
             sch->curr_status.scsw.count = ccw.count - sizeof(indicators);
             ret = 0;