ldpd: Check if the thread is scheduled before calling for remained time

ldpd: Check if the thread is scheduled before calling for remained time

LDPD crashes when hold time is configured to 65535:
(gdb) bt
0  0x00007f8c3fc224bb in raise () from /lib64/libpthread.so.0
1  0x00007f8c4138a3dd in core_handler () from /lib64/libfrr.so.0
2  <signal handler called>
3  0x00007f8c3fc1ccc0 in pthread_mutex_lock () from /lib64/libpthread.so.0
4  0x00007f8c4139914b in thread_timer_remain_msec () from /lib64/libfrr.so.0
5  0x00007f8c41399209 in thread_timer_remain_second () from /lib64/libfrr.so.0
6  0x000000000040eb19 in adj_to_ctl ()
7  0x0000000000427b38 in ldpe_nbr_ctl ()
8  0x000000000042fd68 in control_dispatch_imsg ()
9  0x00007f8c4139a628 in thread_call () from /lib64/libfrr.so.0
10 0x00000000004265fc in ldpe ()
11 0x000000000040a68f in main ()

Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>