missing per-skiplist overheads in MEMORY USAGE

these had severe impact for small zsets, for instance ones with just one
element that is longer than 64 (causing it not to be ziplist encoded)

src/object.c

@@ -834,7 +834,9 @@ size_t objectComputeSize(robj *o, size_t sample_size) {
             d = ((zset*)o->ptr)->dict;
             zskiplist *zsl = ((zset*)o->ptr)->zsl;
             zskiplistNode *znode = zsl->header->level[0].forward;
-            asize = sizeof(*o)+sizeof(zset)+(sizeof(struct dictEntry*)*dictSlots(d));
+            asize = sizeof(*o)+sizeof(zset)+sizeof(zskiplist)+sizeof(dict)+
+                    (sizeof(struct dictEntry*)*dictSlots(d))+
+                    zmalloc_size(zsl->header);
             while(znode != NULL && samples < sample_size) {
                 elesize += sdsAllocSize(znode->ele);
                 elesize += sizeof(struct dictEntry) + zmalloc_size(znode);