Make SGLockingQueue::pop usable.

Since there's no external lock, it was impossible to check the queue was non-empty before a pop, without a race condition. To avoid this, tolerate an empty queue and return a default constructed value.

simgear/threads/SGQueue.hxx

@@ -127,8 +127,8 @@ public:
      */
     virtual T pop() {
 	SGGuard<SGMutex> g(mutex);
-	//if (fifo.empty()) throw NoSuchElementException();
-	assert( ! this->fifo.empty() );
+	    if (this->fifo.empty()) return T(); // assumes T is default constructable
+        
 //  	if (fifo.empty())
 //  	{
 //  	    mutex.unlock();