Game crashed in Sound_Handler

This happened originally in windows when I was porting the code to compile on visual studio.

The game crashed after few minutes to a threading bug in sound_handler.cc:328:
       const std::map<uint32_t, std::string>::const_iterator active_fx_end =
               m_active_fx.end();
       for
               (std::map<uint32_t, std::string>::const_iterator it =
                m_active_fx.begin();
                it != active_fx_end;
                ++it)
       {
       // Crashed here! it is not valid and m_active_fx is empty.
       // Probably threading problem with erase() in handle_channel_finished()
               if (it->second == fx_name) {

iterator is bad, m_active is empty but active_fx_end is != it. Looks
like a threading bug. Sound_Handler::m_active_fx must be protected by
a lock because handle_channel_finished() erases items from another
thread.

Confirmed later that m_active_fx map is actively accessed from multiple threads.