SDL_mixer: Add a NULL-Check to Mix_VolumeChunk()

From 680d8d0866816654f0c0f6d53c64c237c038da0c Mon Sep 17 00:00:00 2001
From: Maxim Atanasov <[EMAIL REDACTED]>
Date: Fri, 14 May 2021 22:25:04 +0200
Subject: [PATCH] Add a NULL-Check to Mix_VolumeChunk()

`Mix_VolumeChunk()` doesn't check if the chunk passed to it is NULL. If that happens, a crash with a segfault will follow.
Would returning `-1` as a value for the previous volume be acceptable?
---
 src/mixer.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/src/mixer.c b/src/mixer.c
index 9936a0d..b9bdab7 100644
--- a/src/mixer.c
+++ b/src/mixer.c
@@ -1083,9 +1083,11 @@ int Mix_Volume(int which, int volume)
 /* Set volume of a particular chunk */
 int Mix_VolumeChunk(Mix_Chunk *chunk, int volume)
 {
-    int prev_volume;
-
-    prev_volume = chunk->volume;
+    if (chunk == NULL) {
+        return(-1);
+    }
+  
+    int prev_volume = chunk->volume;
     if (volume >= 0) {
         if (volume > MIX_MAX_VOLUME) {
             volume = MIX_MAX_VOLUME;