If reading the users config fails, say because $HOME is unreadable,
provide an empty config rather than a NULL config.  This fixes a
SEGV and allows command line options that override the default
config to work.

* subversion/svn/svn.c
  (sub_main): Provide a fallback empty config.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1590751 13f79535-47bb-0310-9956-ffa450edef68