SDL: Fix uninitialized length in X11_GetClipboardData causing test failures (#14322)

SDL Commits
1 
From 093fbfd867a574359ea0d987adb78cdda935688c Mon Sep 17 00:00:00 2001
From: DONGGEUN YOO <[EMAIL REDACTED]>
Date: Sat, 25 Oct 2025 10:41:58 -0400
Subject: [PATCH] Fix uninitialized length in X11_GetClipboardData causing test
 failures (#14322)

---
 src/video/x11/SDL_x11clipboard.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/src/video/x11/SDL_x11clipboard.c b/src/video/x11/SDL_x11clipboard.c
index f3e9f372ca47c..7f083c3f5f2f6 100644
--- a/src/video/x11/SDL_x11clipboard.c
+++ b/src/video/x11/SDL_x11clipboard.c
@@ -273,6 +273,9 @@ bool X11_SetClipboardData(SDL_VideoDevice *_this)
 void *X11_GetClipboardData(SDL_VideoDevice *_this, const char *mime_type, size_t *length)
 {
     SDL_VideoData *videodata = _this->internal;
+
+    *length = 0;
+
     if (!SDL_HasInternalClipboardData(_this, mime_type)) {
         // This mime type wasn't advertised by the last selection owner.
         // The atom might still have data, but it's stale, so ignore it.