Fix bug in utf82u_strlen(). Count more in utf8_strlen() and utf82u_strlen()

short is only {0..0xffff}. Unlikely to encounter large strings but if yes,
then count further, howeverm stop counting if len overflows.
Merge pull request #868 from JoesCat/master

Fix bug in utf82u_strlen(). Count more in utf8_strlen() and utf82u_strle...