s390-virtio: Check for NULL device in reset hypercall

s390_virtio_bus_find_mem() may return a NULL VirtIOS390Device.
If called with, e.g., args[0] == 0, this leads to a segfault.
Fix this by adding error handling as done for other hypercalls.

Present since baf0b55a (Implement
virtio reset).
Signed-off-by: NAndreas Färber <afaerber@suse.de>
Signed-off-by: NAlexander Graf <agraf@suse.de>

hw/s390x/s390-virtio.c

@@ -86,6 +86,9 @@ static int s390_virtio_hcall_reset(const uint64_t *args)
     VirtIOS390Device *dev;
 
     dev = s390_virtio_bus_find_mem(s390_bus, mem);
+    if (dev == NULL) {
+        return -EINVAL;
+    }
     virtio_reset(dev->vdev);
     stb_phys(dev->dev_offs + VIRTIO_DEV_OFFS_STATUS, 0);
     s390_virtio_device_sync(dev);