mod_dav: Keep track of length of cdata properly when removing leading spaces.

* modules/dav/main/util.c
  (dav_xml_get_cdata): reduce len variable when increasing cdata pointer.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1556428 13f79535-47bb-0310-9956-ffa450edef68