merge revision(s) 4bff8e84232594ecb9914e2a8437b7c40a63b799: [Backport #16814]

	Ensure that the head of the vacancy list is correctly inserted into
	 the linked list.

	See <https://bugs.ruby-lang.org/issues/16814> for more details.
	---
	 cont.c | 1 +
	 1 file changed, 1 insertion(+)

1 files changed, 1 insertions, 0 deletions

diff --git a/cont.c b/cont.c
index 0c49d34742..7f971a9e9d 100644
--- a/cont.c
+++ b/cont.c
@@ -324,6 +324,7 @@ fiber_pool_vacancy_push(struct fiber_pool_vacancy * vacancy, struct fiber_pool_v
 #ifdef FIBER_POOL_ALLOCATION_FREE
     if (head) {
         head->previous = vacancy;
+        vacancy->previous = NULL;
     }
 #endif