Change max byte length of UTF-8 to 4 bytes

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.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@58954 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

enc/utf_8.c

@@ -417,7 +417,7 @@ get_case_fold_codes_by_str(OnigCaseFoldType flag,
 OnigEncodingDefine(utf_8, UTF_8) = {
   mbc_enc_len,
   "UTF-8",     /* name */
-  6,           /* max byte length */
+  4,           /* max byte length */
   1,           /* min byte length */
   is_mbc_newline,
   mbc_to_code,