iterator: assert tree_iterator has a frame

Although a `tree_iterator` that failed to be properly created
does not have a frame, all other `tree_iterator`s should.  Do not
call `pop` in the failure case, but assert that in all other
cases there is a frame.

src/iterator.c

@@ -558,7 +558,9 @@ static bool tree_iterator__pop_frame(tree_iterator *ti, bool final)
 {
 	tree_iterator_frame *tf = ti->head;
 
-	if (!tf || !tf->up)
+	assert(tf);
+
+	if (!tf->up)
 		return false;
 
 	ti->head = tf->up;
@@ -581,8 +583,9 @@ static void tree_iterator__pop_all(tree_iterator *ti, bool to_end, bool final)
 	while (tree_iterator__pop_frame(ti, final)) /* pop to root */;
 
 	if (!final) {
-		if(ti->head)
-			ti->head->current = to_end ? ti->head->n_entries : 0;
+		assert(ti->head);
+
+		ti->head->current = to_end ? ti->head->n_entries : 0;
 		ti->path_ambiguities = 0;
 		git_buf_clear(&ti->path);
 	}
@@ -774,11 +777,12 @@ static void tree_iterator__free(git_iterator *self)
 {
 	tree_iterator *ti = (tree_iterator *)self;
 
-	tree_iterator__pop_all(ti, true, false);
-
-	if(ti->head)
+	if (ti->head) {
+		tree_iterator__pop_all(ti, true, false);
 		git_tree_free(ti->head->entries[0]->tree);
-	git__free(ti->head);
+		git__free(ti->head);
+	}
+
 	git_pool_clear(&ti->pool);
 	git_buf_free(&ti->path);
 }