Fix SkyDNS crash when Updating
Fix SkyDNS crash when Updating

Sometimes the callback is called without machines. If so don't update
the client, as this crashes SkyDNS and can leave it without machines.

Closes #81

netf/skydns,tuannvm/skydns,AdoHe/skydns,tonicbupt/skydns,hw-qiaolei/skydns,erimatnor/skydns-kubernetes,smarterclayton/skydns,zhang0137/skydns,vinhhrv/skydns,andrewrothstein/skydns,weekface/skydns,ZeaLoVe/skydns,no2key/skydns,zvelo/zvelo-skydns,yonglehou/skydns,loveshell/skydns,skynetservices/skydns,nicescale/skydns