In enc/utf_8.c, change maximum byte length of UTF-8 to 4 bytes (from 6)
to conform to definition of UTF-8. This closes issue #13590.
(This is a retry of r58954, after issue #13590 has been addressed.)
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@58965 b2dd03c8-39d4-4d8f-98ff-823fe69b080e