btrfs-progs: check: change commit condition in fixup_extent_refs()

This patch fixes potential bugs in fixup_extent_refs(). If
btrfs_start_transaction() fails in some way and returns error ptr, It
goes to out logic. But old code checkes whether it is null and it calls
commit. This patch solves the problem with make that it calls only if
ret is no error.

Issue: #409
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Sidong Yang <realwakka@gmail.com>
Signed-off-by: David Sterba <dsterba@suse.com>

check/main.c

@@ -7773,7 +7773,7 @@ static int fixup_extent_refs(struct cache_tree *extent_cache,
 			goto out;
 	}
 out:
-	if (trans) {
+	if (!ret && !IS_ERR(trans)) {
 		int err = btrfs_commit_transaction(trans, gfs_info->tree_root);
 
 		if (!ret)