[ostree] core: Fix possible crash in ostree_mutable_tree_walk()

[Matthew Barnes <mbarnes src gnome org>]

commit d667ebe1568591bdd3c3cc23d751ffc5c0a3752c
Author: Sam Thursfield <ssssam gmail com>
Date:   Tue Mar 31 12:59:43 2015 -0400

    core: Fix possible crash in ostree_mutable_tree_walk()
    
    If the starting index is beyond the end of the list, it's a programming
    error. Previously, the code was trying to raise a runtime error, but
    actually causing a segfault.
    
    This was detected by test code in test-mutable-tree.c, which is removed
    in this commit because it should now not be possible to crash here.
    
    https://bugzilla.gnome.org/747032

 src/libostree/ostree-mutable-tree.c |    8 +++-----
 tests/test-mutable-tree.c           |    7 -------
 2 files changed, 3 insertions(+), 12 deletions(-)
---
diff --git a/src/libostree/ostree-mutable-tree.c b/src/libostree/ostree-mutable-tree.c
index 26a1ca4..17bcdc9 100644
--- a/src/libostree/ostree-mutable-tree.c
+++ b/src/libostree/ostree-mutable-tree.c
@@ -315,11 +315,9 @@ ostree_mutable_tree_walk (OstreeMutableTree     *self,
                           OstreeMutableTree    **out_subdir,
                           GError               **error)
 {
-  if (start >= split_path->len)
-    {
-      return set_error_noent (error, (char*)split_path->pdata[start]);
-    }
-  else if (start == split_path->len - 1)
+  g_return_val_if_fail (start < split_path->len, FALSE);
+
+  if (start == split_path->len - 1)
     {
       *out_subdir = g_object_ref (self);
       return TRUE;
diff --git a/tests/test-mutable-tree.c b/tests/test-mutable-tree.c
index ef2b6b5..b0c2386 100644
--- a/tests/test-mutable-tree.c
+++ b/tests/test-mutable-tree.c
@@ -70,13 +70,6 @@ test_mutable_tree_walk (void)
 
   {
     gs_unref_object OstreeMutableTree *subdir = NULL;
-    g_assert_false (ostree_mutable_tree_walk (tree, split_path, 10, &subdir, &error));
-    g_assert_null (subdir);
-    g_clear_error (&error);
-  }
-
-  {
-    gs_unref_object OstreeMutableTree *subdir = NULL;
     gs_unref_object OstreeMutableTree *a = NULL;
     gs_free char *source_checksum = NULL;
     ostree_mutable_tree_lookup (tree, "a", &source_checksum, &a, &error);