It was possible to end up with a dangling pointer in m_listStack.
This is now avoided by using QPointer and doing nullptr checks before
accessing any QTextList pointer stored there.
We have 2 specimens of garbage that caused crashes before; now they don't.
But only fuzz20450 triggered the dangling pointer in the list stack.
The crash caused by fuzz20580 was fixed by updating md4c from upstream:
4b0fc03077
Change-Id: I8e1eca23b281256a03aea0f55e9ae20f1bdd2a38
Reviewed-by: Robert Loehning <robert.loehning@qt.io>
7447e2b337