Fix potential bug with recursing too many times duing gc

src/refcount.c

@@ -658,19 +658,20 @@ static ptrdiff_t check_gc_root(RefcountContext *ctx, RefcountList **root_ptr) {
             continue;
         }
         uintptr_t count;
+        queue = refcount_list_pop_full(queue, NULL, &ctx->alloc);
         if (ht_has(counts, obj)) {
             count = HT_UUNSTUFF(ht_get(counts, obj));
         } else {
             count = REFCOUNT_OBJECT_ENTRY(ctx, obj)->impl.counted.ref_count;
             ++seen_objects;
+            // don't recuse into objects multiple times
+            obj_held_refs(ctx, obj, &queue);
         }
-        queue = refcount_list_pop_full(queue, NULL, &ctx->alloc);
         if (count > 0) {
             ht_insert(counts, obj, HT_STUFF(--count));
             if (count == 0) {
                 ++clear_objects;
             }
-            obj_held_refs(ctx, obj, &queue);
         }
     }
     ptrdiff_t freed_count = 0;

test/test_refcount.c

@@ -144,6 +144,9 @@ int main(int argc, const char **argv) {
     assert(refcount_context_garbage_collect(c) == 0);
     assert(refcount_context_num_refs(c, a_with_destructor) == 1);
     assert(refcount_context_remove_destructor(c, a_with_destructor, &key));
+    assert(refcount_context_ref(c, a_with_destructor));
+    assert(refcount_context_garbage_collect(c) == 0);
+    assert(refcount_context_unref(c, a_with_destructor));
     assert(!refcount_context_is_doing_gc(c));
     ctx_and_flag.should_be_doing_gc = true;
     assert(refcount_context_garbage_collect(c) == 26);