Minor UTF conversion optimization: use the most suitable variant of the UTF-8 checking functions.
Minor UTF conversion optimization: use the most suitable
variant of the UTF-8 checking functions.

* subversion/libsvn_subr/utf.c
  (svn_utf_cstring_from_utf8,
   svn_utf_cstring_from_utf8_ex2): call the cstring variants
   and let them determine the string length as they go

jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion