Bug fix: --use-ewmh-active-win causes wrong focus state in Awesome

- Fix a bug that causes wrong focus detection result in Awesome and
  maybe other window managers, when --use-ewmh-active-win is enabled and
  _NET_ACTIVE_WINDOW changes before the newly-focused window is mapped.

- Fix a typo that causes more than one window to stay focused after a
  window destruction with --use-ewmh-active-win.

- Fix a bug that find_clientwin() incorrectly returns a window when
  window ID 0 is passed to it.

- Check for window ID 0 in update_ewmh_active_win().

src/compton.h

@@ -378,8 +378,11 @@ typedef struct {
   // === Window related ===
   /// Linked list of all windows.
   struct _win *list;
-  /// Current active window. Used by EWMH _NET_ACTIVE_WINDOW focus
-  /// detection.
+  /// Pointer to <code>win</code> of current active window. Used by
+  /// EWMH <code>_NET_ACTIVE_WINDOW</code> focus detection. In theory,
+  /// it's more reliable to store the window ID directly here, just in
+  /// case the WM does something extraordinary, but caching the pointer
+  /// means another layer of complexity.
   struct _win *active_win;
 
   // === Shadow/dimming related ===