Fix build with MSVC 2013

Also add an assert hinted by MSVC code analysis tool.

No functional change.

src/search.cpp

@@ -1669,6 +1669,7 @@ void Thread::idle_loop() {
           Threads.mutex.lock();
 
           assert(searching);
+          assert(activeSplitPoint);
           SplitPoint* sp = activeSplitPoint;
 
           Threads.mutex.unlock();