fix potential deadlock while shutting down

rtsp.c

@@ -36,7 +36,7 @@ static inline int rtsp_playing(void) {
 
 static void rtsp_take_player(void) {
     if (pthread_mutex_trylock(&playing_mutex)) {
-        debug("another thread is playing, shutting it down\n");
+        debug("shutting down playing thread\n");
         // XXX minor race condition between please_shutdown and signal delivery
         please_shutdown = 1;
         pthread_kill(playing_thread, SIGUSR1);

shairport.c

@@ -4,8 +4,11 @@
 #include "rtsp.h"
 #include "mdns.h"
 
-
+static int shutting_down = 0;
 static void shutdown(void) {
+    if (shutting_down)
+        return;
+    shutting_down = 1;
     printf("Shutting down...\n");
     mdns_unregister();
     rtsp_shutdown_stream();