Add fco assertion to fellow_cache_seg_deref()

src/fellow_cache.c

@@ -3573,6 +3573,7 @@ fellow_cache_seg_deref(struct fellow_cache_seg * const *segs, unsigned n)
 	AZ(pthread_mutex_lock(&fco->mtx));
 	while (n--) {
 		fcs = *segs++;
+		assert(fcs->fco == fco);
 		CHECK_OBJ_NOTNULL(fcs, FELLOW_CACHE_SEG_MAGIC);
 		(void) fellow_cache_seg_deref_locked(lcb, fcs);
 	}