Fix heap dump with ractor barrier

When a ractor was being initialized and it would join the heap dump barrier when
allocating its queue or its ports, the heap dump code calls `rb_obj_memsize` on
the ractor and this function assumed `ports` was never NULL. We need to check for
the NULL case in case the ractor is still being initialized. Hopefully other T_DATA
objects don't suffer from the same issue, otherwise we could revert the ractor barrier
during heap dump or not use `rb_obj_memsize` on T_DATA during the heap dump.

Author: luke-gruber

ractor_sync.c

@@ -699,7 +699,12 @@ ractor_sync_free(rb_ractor_t *r)
 static size_t
 ractor_sync_memsize(const rb_ractor_t *r)
 {
-    return st_table_size(r->sync.ports);
+    if (r->sync.ports) {
+        return st_table_size(r->sync.ports);
+    }
+    else {
+        return 0;
+    }
 }
 
 static void