Reset chars_changed_stamp in _gtk_text_btree_unref

Using a TextIter after it has been invalidated by changes to the contents of a TextBuffer causes a helpful warning. However, using it after the TextBuffer has been disposed, but before the TextBuffer's memory is overwritten, does not. I propose that a line tree->chars_changed_stamp = 0; be added to _gtk_text_btree_unref so that this scenario also causes a helpful warning.

After this change, it seems fine for language bindings to treat TextIter records as plain values. Without this change, language bindings have to take special measures to ensure that a live TextIter keeps the TextBuffer alive.