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

tests/test-mutable-tree.c

@@ -68,13 +68,6 @@ test_mutable_tree_walk (void)
     g_clear_error (&error);
   }
 
-  {
-    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;