Fix insufficient struct initialization in init_mouse_location()

See pull request #3252. This is the fix @Vultraz prefers to the PR.

(cherry-picked from commit 7fc6a23)

src/gui/core/event/handler.cpp

@@ -833,7 +833,7 @@ void init_mouse_location()
 {
 	point mouse = get_mouse_position();
 
-	SDL_Event event;
+	SDL_Event event{};
 	event.type = SDL_MOUSEMOTION;
 	event.motion.type = SDL_MOUSEMOTION;
 	event.motion.x = mouse.x;