Previously we used the next character following the found prefix to
determine if the match ended on a broken character.

This had caused surprising behaviour when a valid character was followed
by a UTF-8 continuation byte.

This commit changes the behaviour to instead look for the end of the
last character in the prefix.

[Bug #19784]

Co-authored-by: ywenc <ywenc@github.com>
Co-authored-by: Nobuyoshi Nakada <nobu@ruby-lang.org>