Fix crash on activating a text area without a real keyboard

It will cause on following situation:

* Enable weston-simple-im
  * It's also reproducible with IBus or Fcitx5
* Connect a mouse or a touch panel
* Don't connect keyboad
* Launch weston-editor and touch the text area

It doesn't reproduce with weston-keyboard because it doesn't use
grab_keyboard of input-method-context-v1.

Signed-off-by: Takuro Ashie <ashie@clear-code.com>

compositor/text-backend.c

@@ -676,6 +676,9 @@ input_method_context_grab_keyboard(struct wl_client *client,
 	struct weston_seat *seat = context->input_method->seat;
 	struct weston_keyboard *keyboard = weston_seat_get_keyboard(seat);
 
+	if (!keyboard)
+		return;
+
 	cr = wl_resource_create(client, &wl_keyboard_interface, 1, id);
 	wl_resource_set_implementation(cr, NULL, context, unbind_keyboard);