submodule: set a default to the iterator

The iterator is always freed, but not always initialized first.

src/submodule.c

diff --git a/src/submodule.c b/src/submodule.c
index fc3dcb4..0db58f4 100644
--- a/src/submodule.c
+++ b/src/submodule.c
@@ -401,9 +401,10 @@
 static int submodules_from_head(git_strmap *map, git_tree *head, git_config *cfg)
 {
 	int error;
-	git_iterator *i;
+	git_iterator *i = NULL;
 	const git_index_entry *entry;
 	git_strmap *names = 0;
+
 	git_strmap_alloc(&names);
 	if ((error = load_submodule_names(names, cfg)))
 		goto done;