Currently in Wine TEB pointer obtained by NtQueryInformationThread( ThreadBasicInformation ) from '( get_thread_info )' server call can be NULL when thread was already created in the creating thread ('( new_thread )' was called) but not yet initialized on its own and didn't yet call '( init_thread )'. In that state thread can be found. The blamed commit actually tries to handle such a situation by checking TEB pointer for NULL but for wow64 thread (both in new 64 on 32 and old wow64 modes) the TEB pointer gets offset and becomes 0x2000 instead of NULL, leading to crash in alloc_tls_slot().
I am not entirely sure if it is ever possible to get NULL teb pointer from NtQueryInformationThread() on Windows, probably not. But changing this behaviour is rather involved as the TEB should not only get its address but probably be further initialized before it can be reported from NtQueryInformationThread which happens in the thread itself before it calls '(init_thread)'. I hope this added check is fine to fix the regression and is safe enough for the end of code freeze.