SDL: audio: unset a freed variable to NULL

SDL Commits
1 
From e969160de03e13246bcdab2ccf7e07a78f4421f1 Mon Sep 17 00:00:00 2001
From: "Ryan C. Gordon" <[EMAIL REDACTED]>
Date: Mon, 3 Jul 2023 11:52:01 -0400
Subject: [PATCH] audio: unset a freed variable to NULL

---
 src/audio/SDL_audio.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/src/audio/SDL_audio.c b/src/audio/SDL_audio.c
index cc6f585214a2..8926b7e000c4 100644
--- a/src/audio/SDL_audio.c
+++ b/src/audio/SDL_audio.c
@@ -574,6 +574,7 @@ int SDL_InitAudio(const char *driver_name)
 
         SDL_zero(current_audio);
         SDL_DestroyRWLock(device_list_lock);
+        current_audio.device_list_lock = NULL;
         return -1;  // No driver was available, so fail.
     }