vdpa: Skip the maps not in the iova tree

Next patch will skip the registering of dma maps that the vdpa device
rejects in the iova tree. We need to consider that here or we cause a
SIGSEGV accessing result.

Reported-by: Lei Yang <leiyang@redhat.com>
Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>

hw/virtio/vhost-vdpa.c

@@ -289,6 +289,10 @@ static void vhost_vdpa_listener_region_del(MemoryListener *listener,
         };
 
         result = vhost_iova_tree_find_iova(v->iova_tree, &mem_region);
+        if (!result) {
+            /* The memory listener map wasn't mapped */
+            return;
+        }
         iova = result->iova;
         vhost_iova_tree_remove(v->iova_tree, result);
     }