Fix mbstring support for ISO-2022-JP-MS encoding

- Treat it as error if multi-byte string or escape sequence is truncated
- Don't allow 'control' characters or escape sequences to appear in the middle
  of a multi-byte char

As with ISO-2022-JP-KDDI, the main reference used to develop the tests was
the behavior of the existing code. It would have been better to have some
independent reference which we could cross-check our code against, but I
couldn't find one.

Author: alexdowad