ostream-zlib: Ignore missing finish if parent stream is ignoring errors

This fixes panic with imap_zlib plugin when client enables the IMAP COMPRESS
extension and disconnects:

Panic: file ostream-zlib.c: line 36 (o_stream_zlib_close): assertion failed: (zstream->ostream.finished || zstream->ostream.ostream.stream_errno != 0)

src/lib-compression/ostream-zlib.c

@@ -33,7 +33,8 @@ static void o_stream_zlib_close(struct iostream_private *stream,
 	struct zlib_ostream *zstream = (struct zlib_ostream *)stream;
 
 	i_assert(zstream->ostream.finished ||
-		 zstream->ostream.ostream.stream_errno != 0);
+		 zstream->ostream.ostream.stream_errno != 0 ||
+		 zstream->ostream.error_handling_disabled);
 	(void)deflateEnd(&zstream->zs);
 	if (close_parent)
 		o_stream_close(zstream->ostream.parent);