[bugfix] after save_cont any local variable must not be modified!
[bugfix] after save_cont any local variable must not be modified!

If save_cont returns a cont object, a local variable will receive the
object and the machine stack state will have just been changed then.
To avoid that, we need set cont object to the local before the machine
stack is copied.

dcurrie/picrin,leavesbnw/picrin,ktakashi/picrin,ktakashi/picrin,ktakashi/picrin,leavesbnw/picrin,picrin-scheme/picrin,omasanori/picrin,koba-e964/picrin,dcurrie/picrin,koba-e964/picrin,koba-e964/picrin,picrin-scheme/picrin,leavesbnw/picrin,omasanori/picrin