Fix a segfault occuring when svn_stream_read_full() or svn_stream_skip() are
called for a no-op stream created using svn_stream_create().  Instead of
crashing, these scenarios should gracefully return an expected error
(SVN_ERR_STREAM_NOT_SUPPORTED).

* subversion/libsvn_subr/stream.c
  (svn_stream_create): Do not forget to zero out the 'read_full_fn' member.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1639637 13f79535-47bb-0310-9956-ffa450edef68