Revision 1886853

Following up on r1886774, ensure that when we close the file in the install
stream, the corresponding `file` field is always set to NULL.

In other words, keep the internal state consistent, even though we don't
currently retry calls such as svn_stream__install_stream() or
svn_stream__install_delete().

* subversion/libsvn_subr/stream.c
  (install_stream_close_file): New helper function.  Close the underlying file
   of the install stream and set the corresponding `file` field to NULL.
  (svn_stream__install_finalize,
   svn_stream__install_stream,
   svn_stream__install_delete): Use the new helper to close the file.