Platform: gracefully fail GlfwApplication::exec() if no window was opened

Signed-off-by: Squareys <squareys@googlemail.com>

doc/changelog.dox

@@ -287,6 +287,9 @@ See also:
 -   @ref Platform::GlfwApplication::InputEvent::Modifier::Super is properly
     reported for both left and right modifier (Windows/⌘) key instead of just
     the right one
+-   @ref Platform::GlfwApplication::exec() now asserts instead of crashing if
+    the constructor fails to create a window (see [mosra/magnum#192](https://github.com/mosra/magnum/issues/192),
+    [mosra/magnum#272](https://github.com/mosra/magnum/pull/272))
 
 @subsection changelog-latest-docs Documentation
 

src/Magnum/Platform/GlfwApplication.cpp

@@ -436,6 +436,8 @@ void GlfwApplication::setSwapInterval(const Int interval) {
 }
 
 int GlfwApplication::exec() {
+    CORRADE_ASSERT(_window, "Platform::GlfwApplication::exec(): no window opened", {});
+
     while(!glfwWindowShouldClose(_window)) {
         if(_flags & Flag::Redraw) {
             _flags &= ~Flag::Redraw;