cat-file: fix a minor memory leak in batch_objects

We should always have been freeing our strbuf, but doing so
consistently was annoying until the refactoring in the
previous patch.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/cat-file.c

@@ -304,6 +304,7 @@ static int batch_objects(struct batch_options *opt)
 			break;
 	}
 
+	strbuf_release(&buf);
 	return retval;
 }