buddy storage: Adjust to changed STV API

Ref: https://github.com/varnishcache/varnish-cache/pull/4056

src/buddy_storage.c

@@ -172,6 +172,8 @@ sbu_objallocwithnuke(/*lint -e{818}*/ struct worker *wrk,
 
 	reqs = BUDDY_REQS_STK(buddy, 2);
 
+	if (size == 0)
+		size = (size_t)1 << stvbu->tune.chunk_exponent;
 	assert(size > 0);
 
 	if (flags & LESS_MEM_ALLOCED_IS_OK)
@@ -401,7 +403,12 @@ sbu_getspace(struct worker *wrk, struct objcore *oc, ssize_t *sz,
 	CHECK_OBJ_NOTNULL(wrk, WORKER_MAGIC);
 	AN(sz);
 	AN(ptr);
-	assert(*sz > 0);
+	assert(*sz >= 0);
+
+	// XXX VC >7.4 remove workaround
+	// https://github.com/varnishcache/varnish-cache/pull/4056
+	if (*sz == cache_param->fetch_chunksize)
+		*sz = 0;
 
 	CAST_OBJ_NOTNULL(o, oc->stobj->priv, OBU_MAGIC);
 	CHECK_OBJ_NOTNULL(oc->boc, BOC_MAGIC);