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.

1 files changed, 1 insertions, 0 deletions

diff --git a/cont.c b/cont.c
index 37a9dc8adf..139c12c693 100644
--- a/cont.c
+++ b/cont.c
@@ -336,6 +336,7 @@ 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