A code pattern `p + enclen(enc, p, pend)` may lead to a buffer overrun
if incomplete bytes of a UTF-8 character is placed at the end of a
string. Because this pattern is used in several places in onigmo,
this change fixes the issue in the side of `enclen`: the function should
not return a number that is larger than `pend - p`.

Co-Authored-By: Nobuyoshi Nakada <nobu@ruby-lang.org>