[PATCH v3 02/19] refs.c: allow passing NULL to ref_transaction_free

Allow ref_transaction_free to be called with NULL and in extension allow
ref_transaction_rollback to be called for a NULL transaction.

This allows us to write code that will
  if ( (!transaction ||
        ref_transaction_update(...))  ||
      (ref_transaction_commit(...) && !(transaction = NULL)) {
          ref_transaction_rollback(transaction);
          ...
  }

In this case transaction is reset to NULL IFF ref_transaction_commit() was
invoked and thus the rollback becomes ref_transaction_rollback(NULL) which
is safe. IF the conditional triggered prior to ref_transaction_commit()
then transaction is untouched and then ref_transaction_rollback(transaction)
will rollback the failed transaction.

Signed-off-by: Ronnie Sahlberg <redacted>
---
 refs.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/refs.c b/refs.c
index 138ab70..2d83ef6 100644
--- a/refs.c
+++ b/refs.c

@@ -3303,6 +3303,9 @@ static void ref_transaction_free(struct ref_transaction *transaction)
 {
 	int i;
 
+	if (!transaction)
+		return;
+
 	for (i = 0; i < transaction->nr; i++)
 		free(transaction->updates[i]);
 

-- 
1.9.1.521.g5dc89fa