branch: handle NULL pointers passed to git_branch_iterator_free()

Signed-off-by: Brodie Rao <brodie@sf.io>

src/branch.c

@@ -181,6 +181,9 @@ void git_branch_iterator_free(git_branch_iterator *_iter)
 {
 	branch_iter *iter = (branch_iter *) _iter;
 
+	if (iter == NULL)
+		return;
+
 	git_reference_iterator_free(iter->iter);
 	git__free(iter);
 }