Fix VisualC integer compiler warnings in UTF8 code.

* subversion/libsvn_subr/utf8proc/utf8proc.c
  (utf8proc_encode_char): explicitly cast to the target type. The range
                          has been ensured by previous checks
* subversion/libsvn_subr/utf8proc.c
  (svn_utf__fuzzy_escape): fix a mixed signedness comparison

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1543630 13f79535-47bb-0310-9956-ffa450edef68