SDL: audio: don't allocate buffer in SDL_SetAudioPostmixCallback for NULL callback.

SDL Commits
1 
From ac88ffb7ea626e726dd7be19bfe6de4156414314 Mon Sep 17 00:00:00 2001
From: "Ryan C. Gordon" <[EMAIL REDACTED]>
Date: Wed, 20 Sep 2023 10:03:02 -0400
Subject: [PATCH] audio: don't allocate buffer in SDL_SetAudioPostmixCallback
 for NULL callback.

---
 src/audio/SDL_audio.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/audio/SDL_audio.c b/src/audio/SDL_audio.c
index 1ec7b854e9f1..ea1139720b76 100644
--- a/src/audio/SDL_audio.c
+++ b/src/audio/SDL_audio.c
@@ -1484,7 +1484,7 @@ int SDL_SetAudioPostmixCallback(SDL_AudioDeviceID devid, SDL_AudioPostmixCallbac
     int retval = 0;
     if (logdev) {
         SDL_AudioDevice *device = logdev->physical_device;
-        if (!device->postmix_buffer) {
+        if (callback && !device->postmix_buffer) {
             device->postmix_buffer = (float *)SDL_aligned_alloc(SDL_SIMDGetAlignment(), device->work_buffer_size);
             if (device->mix_buffer == NULL) {
                 retval = SDL_OutOfMemory();