fix pthread_exit from cancellation handler
fix pthread_exit from cancellation handler

cancellation frames were not correctly popped, so this usage would not
only loop, but also reuse discarded and invalid parts of the stack.

chardi/musl,rofl0r/musl-old,davidlazar/musl,chardi/musl,chardi/musl,rofl0r/musl-old,davidlazar/musl