cont.c (rb_fiber_atfork): th->root_fiber may not exist

Otherwise, bootstraptest/test_fork.rb fails with -DVM_CHECK_MODE=2

[Bug #15041]

Fixes: r64589 "cont.c: set th->root_fiber to current fiber at fork"

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64593 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

1 files changed, 6 insertions, 4 deletions

diff --git a/cont.c b/cont.c
index 3f0a5cb45f..7bc1724176 100644
--- a/cont.c
+++ b/cont.c
@@ -1981,11 +1981,13 @@ fiber_to_s(VALUE fibval)
 void
 rb_fiber_atfork(rb_thread_t *th)
 {
-    if (&th->root_fiber->cont.saved_ec != th->ec) {
-        th->root_fiber = th->ec->fiber_ptr;
-        th->root_fiber->cont.type = ROOT_FIBER_CONTEXT;
+    if (th->root_fiber) {
+        if (&th->root_fiber->cont.saved_ec != th->ec) {
+            th->root_fiber = th->ec->fiber_ptr;
+            th->root_fiber->cont.type = ROOT_FIBER_CONTEXT;
+        }
+        th->root_fiber->prev = 0;
     }
-    th->root_fiber->prev = 0;
 }
 #endif