xwayland: Release keys on keyboard `enter` event if `leave` wasn't received

The code here assumed a leave event always occurs between two enter events. On Sway (and presumably other compositors) this happens even if the client has destroyed the wl_surface, but the client gets a null surface here. (Which presumably on on the wire is the id of the destroyed surface.)

This seems like a bad thing to rely on, and is easy to avoid. But if this is correct to assume, the Wayland protocol should be explicit about this.