surface_add_association: properly handle failure to add association to surface.

Remove association buffers if adding did not work properly.

src/xvba_video.c

@@ -215,6 +215,8 @@ int surface_add_association(
             if (obj_surface->assocs[i]->subpicture == assoc->subpicture) {
                 /* XXX: this should not happen, but replace it in the interim */
                 ASSERT(obj_surface->assocs[i]->surface == assoc->surface);
+                SubpictureAssociation *a = obj_surface->assocs[i];
+                free(a); /* XXX: release previous association */
                 obj_surface->assocs[i] = assoc;
                 return 0;
             }