redis-benchmark: Fix broken protocol when used with -a or --dbnum (#8486)

Fix the pointers to the slot hash tags in the case of prefixed commands usage
i.e. AUTH / SELECT

It adjusts the pointers to the slot hash tags in the case of prefixed commands
usage as soon as we get the 1st reply (same like we already did for the random
strings within the command )

(cherry picked from commit efccd6353b)

src/redis-benchmark.c

@@ -527,6 +527,9 @@ static void readHandler(aeEventLoop *el, int fd, void *privdata, int mask) {
                         * we need to randomize. */
                         for (j = 0; j < c->randlen; j++)
                             c->randptr[j] -= c->prefixlen;
+                        /* Fix the pointers to the slot hash tags */
+                        for (j = 0; j < c->staglen; j++)
+                            c->stagptr[j] -= c->prefixlen;
                         c->prefixlen = 0;
                     }
                     continue;