ntdll: Ignore invalid exit_frame when exiting thread.

[Sebastian Lackner]

On 18.04.2016 12:27, Alexandre Julliard wrote:
> Sebastian Lackner <sebastian at fds-team.de> writes:
> 
>> Signed-off-by: Sebastian Lackner <sebastian at fds-team.de>
>> ---
>>
>> When applications switch to a custom stack, exit_frame is not necessarily
>> valid anymore.
> 
> How does it become invalid? Does the app unmap the previous stack?
> 

Yes, MSYS2/Cygwin will switch to its own stack and deallocate the old one.
This patch alone is not sufficient because there are still a couple of other
references to the old stack (like the debug_info struct in start_thread).

If preferred, this issue can also be solved differently by calculting exit_frame
dynamically based on StackBase (after a couple of other bugs are fixed).
Disadvantage is that if applications mess around with StackBase, I'm not sure if
we should trust that the new stack is set up properly.