Check if keyboard_driver is installed before using it in _al_win_kbd_update_shifts().

The function _handle_key_press() cannot be used if keyboard_driver is
NULL and this can happen in the creation process of the window if the user
is pressing keys.

src/allegro/src/keyboard.c

@@ -432,6 +432,8 @@ static INLINE void update_shifts(void)
  */
 void _handle_key_press(int keycode, int scancode)
 {
+   ASSERT(keyboard_driver);
+
    if ((keyboard_driver->poll) || (!keyboard_polled)) {
       /* process immediately */
       if (scancode > 0) {

src/allegro/src/win/wkeybd.c

@@ -100,6 +100,9 @@ static const unsigned char hw_to_mycode[256] =
  */
 void _al_win_kbd_update_shifts(void)
 {
+   if (!keyboard_driver)
+      return;
+
 #define HANDLE_KEY(mycode, vk)                  \
    if (GetAsyncKeyState(vk) & 0x8000) {         \
       if (!key[mycode])                         \