player_control: added player_control.thread

player_control.thread contains the handle of the player thread, or
NULL if the player thread isn't running.

src/player_control.c

@@ -90,7 +90,11 @@ void playerWait(void)
 
 void playerKill(void)
 {
+	assert(pc.thread != NULL);
+
 	player_command(PLAYER_COMMAND_EXIT);
+	g_thread_join(pc.thread);
+	pc.thread = NULL;
 
 	idle_add(IDLE_PLAYER);
 }