change streamAppendItem to use raxEOF instead of raxNext (#9138)

The call to raxNext didn't really progress in the rax, since we were already on the last item.
instead, all it does is check that it is indeed a valid item, so the new code clearer.

src/t_stream.c

@@ -458,8 +458,8 @@ int streamAppendItem(stream *s, robj **argv, int64_t numfields, streamID *added_
     size_t lp_bytes = 0;        /* Total bytes in the tail listpack. */
     unsigned char *lp = NULL;   /* Tail listpack pointer. */
 
-    /* Get a reference to the tail node listpack. */
-    if (raxNext(&ri)) {
+    if (!raxEOF(&ri)) {
+        /* Get a reference to the tail node listpack. */
         lp = ri.data;
         lp_bytes = lpBytes(lp);
     }