Check that we actually have some headers before trying to access them,
preventing a segfault if we get an unexpected response while chatting
with the server.

This could happen if the server is replying normally to the initial
requests but suddenly send a 301 Moved permanently reply.

This happens for ASFs incubator-repo at the time of writing due to some
rewrite magic in their server config.

Reported/discussed on users@: 
https://lists.apache.org/thread/konv626mmdpqk8zms7smqk6qfdbfl1wg

* subversion/libsvn_ra_serf/util.c
  (svn_ra_serf__expect_empty_body): As above



git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1929793 13f79535-47bb-0310-9956-ffa450edef68