This issue was only exposed by ba49345705. The ID pointer of the
material's parent tree-element wasn't actually pointing to an ID, but to
the list-base containing the IDs. It was just unlikely to cause issues
in practice, although an assert was thrown.
Just don't do anything if the object or object-data to unlink the
material from could not be found. The following commit will introduce a
error message about this.
e772087ed6