if_vxlan(4): Invoke vxlan_stop event handler only when the interface is configured

It is harmless but pointless to invoke vxlan_stop event handler when the
interface was not previously configured. This change will also prevent
an assert panic from t4_vxlan_stop_handler().

Reviewed by: kib
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D48494