Fix saving of zero-length lists.

Normally in modern Redis you can't create zero-len lists, however it's
possible to load them from old RDB files generated, for instance, using
Redis 2.8 (see issue #4409). The "Right Thing" would be not loading such
lists at all, but this requires to hook in rdb.c random places in a not
great way, for a problem that is at this point, at best, minor.

Here in this commit instead I just fix the fact that zero length lists,
materialized as quicklists with the first node set to NULL, were
iterated in the wrong way while they are saved, leading to a crash.

The other parts of the list implementation are apparently able to deal
with empty lists correctly, even if they are no longer a thing.

src/rdb.c

@@ -656,7 +656,7 @@ ssize_t rdbSaveObject(rio *rdb, robj *o) {
             if ((n = rdbSaveLen(rdb,ql->len)) == -1) return -1;
             nwritten += n;
 
-            do {
+            while(node) {
                 if (quicklistNodeIsCompressed(node)) {
                     void *data;
                     size_t compress_len = quicklistGetLzf(node, &data);
@@ -666,7 +666,8 @@ ssize_t rdbSaveObject(rio *rdb, robj *o) {
                     if ((n = rdbSaveRawString(rdb,node->zl,node->sz)) == -1) return -1;
                     nwritten += n;
                 }
-            } while ((node = node->next));
+                node = node->next;
+            }
         } else {
             serverPanic("Unknown list encoding");
         }