vm: don't allocate another stack for entry scripts

vm.New() creates a new stack, then we load an entry script with
LoadScriptWithFlags and it creates another one which doesn't make much
sense. rvcount is -1 for it, so all elements are to be copied down anyway and
it's clear so a new loaded script can't dig down to anything it shouldn't be
able to.

pkg/vm/vm.go

@@ -317,7 +317,9 @@ func (v *VM) loadScriptWithCallingHash(b []byte, exe *nef.File, caller util.Uint
 
 	v.checkInvocationStackSize()
 	ctx := NewContextWithParams(b, rvcount, offset)
+	if rvcount != -1 || v.estack.Len() != 0 {
 		v.estack = newStack("evaluation", &v.refs)
+	}
 	ctx.estack = v.estack
 	initStack(&ctx.tryStack, "exception", nil)
 	ctx.callFlag = f