Zero any fields not explicitly filled in.

src/video/x11/SDL_x11events.c

@@ -381,6 +381,7 @@ X11_DispatchEvent(_THIS)
                 req->requestor, req->target);
 #endif
 
+            SDL_zero(sevent);
             sevent.xany.type = SelectionNotify;
             sevent.xselection.selection = req->selection;
             sevent.xselection.target = None;

src/video/x11/SDL_x11window.c

@@ -972,6 +972,7 @@ X11_SetWindowMaximized(_THIS, SDL_Window * window, SDL_bool maximized)
     if (X11_IsWindowMapped(_this, window)) {
         XEvent e;
 
+        SDL_zero(e);
         e.xany.type = ClientMessage;
         e.xclient.message_type = _NET_WM_STATE;
         e.xclient.format = 32;