BUG/MEDIUM: fd/threads: fix incorrect thread selection in wakeup broadcast

BUG/MEDIUM: fd/threads: fix incorrect thread selection in wakeup broadcast

In commit cfdd20a0b ("MEDIUM: fd: support broadcasting updates for foreign
groups in updt_fd_polling") we decided to pick a random thread number among
a set of candidates for a wakeup in case we need an instant change. But the
thread count range was wrong (MAX_THREADS) instead of tg->count, resulting
in random crashes when thread groups are > 1 and MAX_THREADS > 64.

No backport is needed, this was introduced in 2.7-dev2.