vhost: fix virtqueue access lock check for handlers

As the vhost library may receive a request for an unsupported request,
it is necessary to check msg_handler before checking if locking queue
pairs is requested.

Coverity issue: 415049
Fixes: 5e8fcc6 ("vhost: enhance virtqueue access lock asserts")

Signed-off-by: David Marchand <[email protected]>
Acked-by: Kevin Traynor <[email protected]>
Reviewed-by: Maxime Coquelin <[email protected]>

Author: david-marchand

lib/vhost/vhost_user.c

@@ -3171,7 +3171,7 @@ vhost_user_msg_handler(int vid, int fd)
 	 * inactive, so it is safe. Otherwise taking the access_lock
 	 * would cause a dead lock.
 	 */
-	if (msg_handler->lock_all_qps) {
+	if (msg_handler != NULL && msg_handler->lock_all_qps) {
 		if (!(dev->flags & VIRTIO_DEV_VDPA_CONFIGURED)) {
 			vhost_user_lock_all_queue_pairs(dev);
 			unlock_required = 1;