Avoid the necessity of nulling out svn_stream_t members one by one.  This
should protect us against a possibility of making mistakes similiar to the
one fixed in r1639637.

* subversion/libsvn_subr/stream.c
  (svn_stream_create): Use apr_pcalloc() instead of apr_palloc() and only
   initialize the non-null parts of the svn_stream_t structure; that's just
   the STREAM->BATON.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1639644 13f79535-47bb-0310-9956-ffa450edef68