lib-fs: fs-metawrap - Don't try to write to an already finished ostream

Fixes a crash after recent changes when trying to write an empty file via
fs-metawrap.

src/lib-fs/fs-metawrap.c

@@ -391,10 +391,16 @@ static int fs_metawrap_write_stream_finish(struct fs_file *_file, bool success)
 	}
 	/* finish writing the temporary file */
 	if (file->temp_output->offset == 0) {
-		/* empty file */
-		fs_metawrap_write_metadata_to(file, file->temp_output);
+		/* empty file - temp_output is already finished,
+		   so we can't write to it. */
+		string_t *str = t_str_new(128);
+
+		o_stream_destroy(&file->temp_output);
+		fs_metawrap_append_metadata(file, str);
+		input = i_stream_create_copy_from_data(str_data(str), str_len(str));
+	} else {
+		input = iostream_temp_finish(&file->temp_output, IO_BLOCK_SIZE);
 	}
-	input = iostream_temp_finish(&file->temp_output, IO_BLOCK_SIZE);
 	if (file->metadata_changed_since_write) {
 		/* we'll need to recreate the metadata. do this by creating a
 		   new istream combining the new metadata header and the