Fix js error in resize instance form
Fix js error in resize instance form

If an empty new flavor selected again, the disk will be
undefined, we'll see the following in console.log.

1c302c01cae3.js:208 Uncaught TypeError:
Cannot read property 'disk' of undefined

Change-Id: Idd93a3f352386f85bfd947ad4c948bb3aa69e04e
Closes-Bug: #1682788

NeCTAR-RC/horizon,NeCTAR-RC/horizon,noironetworks/horizon,BiznetGIO/horizon,noironetworks/horizon,ChameleonCloud/horizon,NeCTAR-RC/horizon,openstack/horizon,BiznetGIO/horizon,ChameleonCloud/horizon,BiznetGIO/horizon,yeming233/horizon,openstack/horizon,yeming233/horizon,noironetworks/horizon,BiznetGIO/horizon,yeming233/horizon,ChameleonCloud/horizon,noironetworks/horizon,openstack/horizon,ChameleonCloud/horizon,yeming233/horizon,NeCTAR-RC/horizon,openstack/horizon