Merge pull request #11626 from yuyichao/module-parent

Only visit module parent when it is not NULL

Author: carnaval

src/gc.c

@@ -1567,7 +1567,9 @@ NOINLINE static int gc_mark_module(jl_module_t *m, int d)
         refyoung |= gc_push_root(m->constant_table, d);
     }
 
-    refyoung |= gc_push_root(m->parent, d);
+    if (m->parent) {
+        refyoung |= gc_push_root(m->parent, d);
+    }
 
     return refyoung;
 }