A fix and a test case for gh-131

Fix a hang on box.fiber.resume() of an attached fiber which gets
cancelled.
When the child fiber got cancelled before detach, the parent
never was woken up, and would hang forever.
Wake up the parent, and, in the parent, do not try to access
a dead fiber.
Merge remote-tracking branch 'origin/stable'

Conflicts:
	src/lua/init.cc
	test/box/fiber.result
	test/box/fiber.test