MT-Cannon: Assert that current thread stack is non-empty

[Inphi]

For all MT-Cannon state transitions, a post condition must be that the current thread stack is non-empty. This is a check the VM should enforce. Every thread stack operation, including push, pop and preempt, require a thread to exist. And this assumption is maintained after the operation.

The only exception to this is when the program is exiting. That is either when the last thread exists or when sys_exit_group is invoked.