Platform: ensure Sdl2Application has a window opened in the main loop.

Same is done in GlfwApplication, and not doing so makes no sense.

src/Magnum/Platform/Sdl2Application.cpp

@@ -812,6 +812,12 @@ void Sdl2Application::exit(const int exitCode) {
 }
 
 bool Sdl2Application::mainLoopIteration() {
+    #ifndef CORRADE_TARGET_EMSCRIPTEN
+    CORRADE_ASSERT(_window, "Platform::Sdl2Application::mainLoopIteration(): no window opened", {});
+    #else
+    CORRADE_ASSERT(_surface, "Platform::Sdl2Application::mainLoopIteration(): no window opened", {});
+    #endif
+
     #ifndef CORRADE_TARGET_EMSCRIPTEN
     const UnsignedInt timeBefore = _minimalLoopPeriod ? SDL_GetTicks() : 0;
     #endif