Fix a simple to trigger, but never reported segfault in the auth helper code.

* subversion/include/svn_auth.h
  (SVN_AUTH_PARAM_STORE_PLAINTEXT_PASSWORDS,
   SVN_AUTH_PARAM_DONT_STORE_SSL_CLIENT_CERT_PP,
   SVN_AUTH_PARAM_STORE_SSL_CLIENT_CERT_PP_PLAINTEXT): Mark as new in 1.6.

* subversion/libsvn_subr/simple_providers.c
  (svn_auth__simple_creds_cache_set): If nobody has set
     SVN_AUTH_PARAM_STORE_PLAINTEXT_PASSWORDS (introduced in 1.6),
     we shouldn't segfault.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1443556 13f79535-47bb-0310-9956-ffa450edef68