[libhilti] Correct nullpointer hlt_timer_cancel

On hlt_execution_context_delete, map dtor executes hlt_timer_cancel
that calls priority_queue_remove with *mgr->timers->d equals to 0, this
produces a segfault ( getpri(q->[posn] = 0x0) ).
Fix this bug by checking *mgr->timers->d value before the call

libhilti/timer.c

@@ -197,7 +197,8 @@ void hlt_timer_cancel(hlt_timer* timer, hlt_exception** excpt, hlt_execution_con
         return;
     }
 
-    priority_queue_remove(timer->mgr->timers, timer);
+   if (*timer->mgr->timers->d != 0)
+        priority_queue_remove(timer->mgr->timers, timer);
     GC_DTOR(timer, hlt_timer, ctx);
 
     timer->mgr = 0;