Make missing write_fn on stream be symmetric with a missing read_fn.

* subversion/libsvn_subr/stream.c
  (svn_stream_write): Return SVN_ERR_STREAM_NOT_SUPPORTED instead of an
    assertion.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1568567 13f79535-47bb-0310-9956-ffa450edef68