Fix use of an unitialised variable identified by valgrind.

* subversion/libsvn_ra_serf/commit.c
  (setup_if_header_recursive): Always initialise return value.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1559197 13f79535-47bb-0310-9956-ffa450edef68