SDL_image: Fix heap underflow WRITE in XCF read_string (CWE-787)

SDL Commits
1 
From becd2b6fb242ba9ac60c66a4f3f77d1849aa2a02 Mon Sep 17 00:00:00 2001
From: Jorge Barredo Ferreira <[EMAIL REDACTED]>
Date: Mon, 6 Apr 2026 19:30:06 +0200
Subject: [PATCH] Fix heap underflow WRITE in XCF read_string (CWE-787)

When string length is 0, data[-1] writes before heap allocation.
Guard tmp == 0 before the null-terminator write.
---
 src/IMG_xcf.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/src/IMG_xcf.c b/src/IMG_xcf.c
index f67817a0..dac76c3c 100644
--- a/src/IMG_xcf.c
+++ b/src/IMG_xcf.c
@@ -237,6 +237,13 @@ static char *read_string(SDL_IOStream *src)
     char *data = NULL;
 
     if (SDL_ReadU32BE(src, &tmp)) {
+        if (tmp == 0) {
+            data = (char *) SDL_malloc(1);
+            if (data) {
+                data[0] = 0;
+            }
+            return data;
+        }
         remaining = SDL_GetIOSize(src) - SDL_TellIO(src);
         if (tmp <= remaining) {
             data = (char *)SDL_malloc(tmp);