merge revision(s) 65ae2bb2e045aa8b668d3c30515f5a6cb3eb68ad: [Backport #18902]

	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(+)

1 files changed, 4 insertions, 0 deletions

diff --git a/thread.c b/thread.c
index 9476a7986a..de29340713 100644
--- a/thread.c
+++ b/thread.c
@@ -1364,6 +1364,10 @@ thread_value(VALUE self)
 {
     rb_thread_t *th = rb_thread_ptr(self);
     thread_join(th, Qnil, 0);
+    if (th->value == Qundef) {
+        // If the thread is dead because we forked th->value is still Qundef.
+        return Qnil;
+    }
     return th->value;
 }