Platform: disallow calling framebufferSize() / windowSize() w/o a window.

src/Magnum/Platform/GlfwApplication.cpp

@@ -426,6 +426,8 @@ GlfwApplication::~GlfwApplication() {
 }
 
 Vector2i GlfwApplication::windowSize() const {
+    CORRADE_ASSERT(_window, "Platform::GlfwApplication::windowSize(): no window opened", {});
+
     Vector2i size;
     glfwGetWindowSize(_window, &size.x(), &size.y());
     return size;

src/Magnum/Platform/Sdl2Application.cpp

@@ -514,8 +514,10 @@ bool Sdl2Application::tryCreate(const Configuration& configuration, const GLConf
 Vector2i Sdl2Application::windowSize() const {
     Vector2i size;
     #ifndef CORRADE_TARGET_EMSCRIPTEN
+    CORRADE_ASSERT(_window, "Platform::Sdl2Application::windowSize(): no window opened", {});
     SDL_GetWindowSize(_window, &size.x(), &size.y());
     #else
+    CORRADE_ASSERT(_glContext, "Platform::Sdl2Application::windowSize(): no window opened", {});
     emscripten_get_canvas_element_size(nullptr, &size.x(), &size.y());
     #endif
     return size;
@@ -524,8 +526,10 @@ Vector2i Sdl2Application::windowSize() const {
 Vector2i Sdl2Application::framebufferSize() const {
     Vector2i size;
     #ifndef CORRADE_TARGET_EMSCRIPTEN
+    CORRADE_ASSERT(_window, "Platform::Sdl2Application::framebufferSize(): no window opened", {});
     SDL_GL_GetDrawableSize(_window, &size.x(), &size.y());
     #else
+    CORRADE_ASSERT(_glContext, "Platform::Sdl2Application::framebufferSize(): no window opened", {});
     emscripten_get_canvas_element_size(nullptr, &size.x(), &size.y());
     #endif
     return size;

src/Magnum/Platform/Sdl2Application.h

@@ -927,7 +927,7 @@ class Sdl2Application {
         Vector2 _dpiScaling;
 
         #ifndef CORRADE_TARGET_EMSCRIPTEN
-        SDL_Window* _window;
+        SDL_Window* _window{};
         UnsignedInt _minimalLoopPeriod;
         #else
         Vector2i _lastKnownCanvasSize;
@@ -937,7 +937,7 @@ class Sdl2Application {
         #ifndef CORRADE_TARGET_EMSCRIPTEN
         SDL_GLContext _glContext;
         #else
-        SDL_Surface* _glContext;
+        SDL_Surface* _glContext{};
         #endif
         std::unique_ptr<Platform::GLContext> _context;
         #endif