Thread#value: handle threads killed by a fork

	[Bug #18902]

	When a thread is killed because we forked, the `value` if left
	to `Qundef`. Returning it woudl crash the VM.
	---
	 test/ruby/test_thread.rb | 14 ++++++++++++++
	 thread.c                 |  4 ++++
	 2 files changed, 18 insertions(+)