Fix play_sound function interpreting -65535 as sound 1.

It now correctly treats it as a sound that doesn't exist,
and plays nothing.

src/sounds.hpp

@@ -13,8 +13,8 @@
 
 void init_snd_tool();
 bool sound_going(snd_num_t which_s);
-void play_sound(short which, sf::Time delay = sf::Time());
-void one_sound(short which);
+void play_sound(snd_num_t which, sf::Time delay = sf::Time());
+void one_sound(snd_num_t which);
 
 void clear_sound_memory();