Following up on r1534082, pass exact buffersizes to avoid unneeded
calculations. Also fix converting empty strings, which would be handled
as an error.

* subversion/libsvn_subr/win32_xlate.c
  (svn_subr__win32_utf8_to_utf16,
   svn_subr__win32_utf16_to_utf8): Handle empty string as constant return.
     Use the exact number of characters, instead of characters + 1 for
     anything but allocation of the buffer.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1534088 13f79535-47bb-0310-9956-ffa450edef68