Prevent double-free when doing an emergency bailout from the rendering thread.
Prevent double-free when doing an emergency bailout from the rendering thread.

This caused gecko bug 1326176.

This was caused by the fact that we would null out `stm->thread` when in
fact it was still running, so we would delete `stm->emergency_bailout`
twice, because we would return true from `stop_and_join_thread`.

padenot/cubeb,kinetiknz/cubeb,padenot/cubeb,padenot/cubeb,kinetiknz/cubeb,kinetiknz/cubeb