examples/playmus: Prevent stack smashing on long input strings

In addition, fflush(stdin) is undefined behaviour, so remove it.

(cherry picked from commit ef53736)
(cherry picked from commit 8a0af85)

playmus.c

@@ -73,8 +73,7 @@ void Menu(void)
     char buf[10];
 
     printf("Available commands: (p)ause (r)esume (h)alt volume(v#) > ");
-    fflush(stdin);
-    if (scanf("%s",buf) == 1) {
+    if (fgets(buf, sizeof(buf), stdin)) {
         switch(buf[0]) {
 #if defined(SEEK_TEST)
         case '0': Mix_SetMusicPosition(0); break;