RESP3: fix HELLO map len in Sentinel mode.

See #6160.

src/networking.c

@@ -2444,7 +2444,7 @@ void helloCommand(client *c) {
 
     /* Let's switch to the specified RESP mode. */
     c->resp = ver;
-    addReplyMapLen(c,7);
+    addReplyMapLen(c,6 + !server.sentinel_mode);
 
     addReplyBulkCString(c,"server");
     addReplyBulkCString(c,"redis");