SDL: audio: Make sure we don't write to a NULL pointer.

SDL Commits
1 
From 3e9991b535b0caa25e45879fda4c92bd160e6ac7 Mon Sep 17 00:00:00 2001
From: "Ryan C. Gordon" <[EMAIL REDACTED]>
Date: Mon, 26 Jun 2023 21:26:48 -0400
Subject: [PATCH] audio: Make sure we don't write to a NULL pointer.

(This _probably_ never happens in the current codebase, but just in case.)
---
 src/audio/SDL_audiocvt.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/src/audio/SDL_audiocvt.c b/src/audio/SDL_audiocvt.c
index f9f43ed44441..c33daf6ac91b 100644
--- a/src/audio/SDL_audiocvt.c
+++ b/src/audio/SDL_audiocvt.c
@@ -1119,7 +1119,9 @@ int SDL_ClearAudioStream(SDL_AudioStream *stream)
 
     SDL_LockMutex(stream->lock);
     SDL_ClearDataQueue(stream->queue, (size_t)stream->packetlen * 2);
-    SDL_memset(stream->history_buffer, SDL_GetSilenceValueForFormat(stream->src_spec.format), stream->history_buffer_frames * stream->src_spec.channels * sizeof (float));
+    if (stream->history_buffer != NULL) {
+        SDL_memset(stream->history_buffer, SDL_GetSilenceValueForFormat(stream->src_spec.format), stream->history_buffer_frames * stream->src_spec.channels * sizeof (float));
+    }
     stream->future_buffer_filled_frames = 0;
     stream->flushed = SDL_FALSE;
     SDL_UnlockMutex(stream->lock);