Else as https://github.com/python-greenlet/greenlet/pull/285
demonstrates there can be segmentation faults and crashes due to
exceptions from one greenlet propagating to C stack of another greenlet.

No test here. I've tried to do it, but with gevent (contrary to plain
greenlets), spawning new task only schedules corresponding greenlet to
run in the end of current event loop cycle instead of switching to
created greenlet immediately. With this delaying, it was hard for me to
develop corresponding test in a reasonable time.

Hopefully having the test I've done for greenlet itself + hereby
protection is good enough.

/reviewed-on !17