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

(cherry picked from commit 093fbfd867a574359ea0d987adb78cdda935688c)

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.