When compiling with -O3 gcc detects that 'th' may be uninitialized.
This is true, it happens when prio_find_thread is given
which == PRIO_USER or which == PRIO_PGRP.

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>