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

@@ -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;