Fix a potential segfault in merge.

* subversion/libsvn_client/merge.c
  (find_reintegrate_merge): Move the test for yc_ancestor above the use of
    yc_ancestor to calculate source.ancestral.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1469550 13f79535-47bb-0310-9956-ffa450edef68