Fix https://github.com/tarantool/tarantool/issues/33

Fix a crash when box.fiber.find() is called with no arguments.
Add a check for empty Lua stack.
Add a test.
Merge branch 'stable'

Conflicts:
	src/lua/init.cc
	test/box/fiber.result
	test/box/fiber.test
Merge branch 'master' of github.com:tarantool/tarantool