Windows: Fix panic when calling set_fullscreen(None) (#502)

* Windows: Fix panic for set_fullscreen(None) (#501)

* Add condition to prevent panic

Trying to call set_fullscreen(None) on a window that has never been in
fullscreen mode caused a panic before this change.
The responsible method now simply checks if this precondition is met and
returns (does nothing) otherwise.

* Add entry to CHANGELOG

* Add platform specification to CHANGELOG entry

Forgot to add that the to_fullscreen(None) bugfix is Windows only in
CHANGELOG.

src/platform/windows/window.rs

@@ -425,6 +425,13 @@ impl Window {
 
     unsafe fn restore_saved_window(&self) {
         let window_state = self.window_state.lock().unwrap();
+
+        // 'saved_window_info' can be None if the window has never been
+        // in fullscreen mode before this method gets called.
+        if window_state.saved_window_info.is_none() {
+            return;
+        }
+
         // Reset original window style and size.  The multiple window size/moves
         // here are ugly, but if SetWindowPos() doesn't redraw, the taskbar won't be
         // repainted.  Better-looking methods welcome.