crash in TocBackend::updateItem

[Bernhard Roider]

Hello,

i found a crash in TocBackend::updateItem():
steps to reproduce:
- create a new document
- change the document class to article (AMS)
- change the current layout to theorem, definition, proposition, ....
- try to write something -> boom

in  TocBackend::updateItem() the variable toc_item is invalid and accessing toc_item->par_it_ causes 
the crash.

Bernhard