Ensure wc.db has the permissions allowed by umask.

* subversion/libsvn_wc/wc_db_util.c
  (svn_wc__db_util_open_db): Ensure wc.db exists before it is opened by SQLite.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1431279 13f79535-47bb-0310-9956-ffa450edef68