Avoid program crash on audio device fail

src/external/mini_al.c

@@ -3,5 +3,5 @@
 #define MINI_AL_IMPLEMENTATION
 #define MAL_NO_JACK
 #define MAL_NO_OPENAL
-#define MAL_NO_NULL
+//#define MAL_NO_NULL
 #include "mini_al.h"