veyron2/vdl: Fix panic for ANY and OPTIONAL
veyron2/vdl: Fix panic for ANY and OPTIONAL

I think we should be using !v.IsNil() instead of !v.Elem().isNil()
because isNil() is meant to check if the ANY or OPTIONAL contains nil
rather than directly being nil (at least, according to the comments)

Change-Id: Ide3f2635996f778c69c1cee3d4c67daf3aa87d40

vanadium/go.v23,vanadium/go.v23,vanadium/go.v23