Avoid assertion violation when comparing with main-thread

* src/thread.c (unmark_main_thread): New function.
* src/lisp.h (unmark_main_thread): Prototype it.
* src/alloc.c (garbage_collect_1): Call it after sweeping.
(Bug#33073)

* test/src/thread-tests.el (threads-test-bug33073): New test.

1 files changed, 2 insertions, 0 deletions

diff --git a/src/alloc.c b/src/alloc.c
index 6aeac140ca0..1a72941cc92 100644
--- a/src/alloc.c
+++ b/src/alloc.c
@@ -6003,6 +6003,8 @@ garbage_collect_1 (void *end)
   VECTOR_UNMARK (&buffer_defaults);
   VECTOR_UNMARK (&buffer_local_symbols);
 
+  unmark_main_thread ();
+
   check_cons_list ();
 
   gc_in_progress = 0;