Add a couple of asserts to late join

Document and clarify that we cannot rejoin on ourselves
and that we never late join if we are master and all
slaves have finished, inded in this case we exit idle_loop.

No functional change.

src/search.cpp

@@ -1602,6 +1602,9 @@ void Thread::idle_loop() {
                   && sp->slavesCount < MAX_SLAVES_PER_SPLITPOINT
                   && available_to(Threads[i]))
               {
+                  assert(this != Threads[i]);
+                  assert(!(this_sp && this_sp->slavesMask.none()));
+
                   // Compute the recursive split points chain size
                   int level = -1;
                   for (SplitPoint* spp = Threads[i]->activeSplitPoint; spp; spp = spp->parentSplitPoint)