Merge pull request #3940 from take-cheeze/fix_proc_ctx_access

Fix possible memory access error.

src/gc.c

@@ -658,7 +658,7 @@ gc_mark_children(mrb_state *mrb, mrb_gc *gc, struct RBasic *obj)
       struct REnv *e = (struct REnv*)obj;
       mrb_int i, len;
 
-      if (MRB_ENV_STACK_SHARED_P(e) && e->cxt->fib) {
+      if (MRB_ENV_STACK_SHARED_P(e) && e->cxt && e->cxt->fib) {
         mrb_gc_mark(mrb, (struct RBasic*)e->cxt->fib);
       }
       len = MRB_ENV_STACK_LEN(e);