[coap] add IsBound() check in Coap::Send (#4205)
[coap] add IsBound() check in Coap::Send (#4205)


openthread/openthread,srickardti/openthread,chshu/openthread,chshu/openthread,librasungirl/openthread,lanyuwen/openthread,librasungirl/openthread,chshu/openthread,lanyuwen/openthread,bukepo/openthread,librasungirl/openthread,chshu/openthread,abtink/openthread,jwhui/openthread,openthread/openthread,jwhui/openthread,abtink/openthread,srickardti/openthread,openthread/openthread,abtink/openthread,chshu/openthread,librasungirl/openthread,chshu/openthread,lanyuwen/openthread,srickardti/openthread,lanyuwen/openthread,openthread/openthread,jwhui/openthread,lanyuwen/openthread,lanyuwen/openthread,jwhui/openthread,bukepo/openthread,abtink/openthread,srickardti/openthread,bukepo/openthread,bukepo/openthread