Fix scheduler suspend edge case assertion

Prior to this commit, it was possible for a scheduler thread to
suspend even though it wasn't appropriate to do so because
another scheduler thread resumed prior to the lock being
acquired. This would result in incorrect behavior in the
release build and an assertion in the debug build.

See the following link for an example of the assertion being
triggered: https://circleci.com/gh/ponylang/ponyc/3809?utm_campaign=vcs-integration-link&utm_medium=referral&utm_source=github-build-link

This commit fixes the issue by not suspending if the schduler
count changed because another thread resumed.

src/libponyrt/sched/scheduler.c

@@ -407,8 +407,10 @@ static pony_actor_t* suspend_scheduler(scheduler_t* sched,
     memory_order_relaxed);
 
   // make sure the scheduler count didn't change
-  pony_assert(sched_count == current_active_scheduler_count);
-  (void)current_active_scheduler_count;
+  // if it did, then another thread resumed and it may not be
+  // appropriate for us to suspend any longer, so don't suspend
+  if(sched_count != current_active_scheduler_count)
+    return actor;
 
   atomic_store_explicit(&active_scheduler_count, sched_count - 1,
     memory_order_relaxed);