Revert "No longer need to insert write barriers for fibers."

This reverts commit c6736357a7.
The assumption was wrong and caused the issue; fix #4020

src/vm.c

@@ -949,7 +949,12 @@ mrb_vm_run(mrb_state *mrb, struct RProc *proc, mrb_value self, unsigned int stac
   if (c->ci - c->cibase > cioff) {
     c->ci = c->cibase + cioff;
   }
-  mrb->c = c;
+  if (mrb->c != c) {
+    if (mrb->c->fib) {
+      mrb_write_barrier(mrb, (struct RBasic*)mrb->c->fib);
+    }
+    mrb->c = c;
+  }
   return result;
 }
 
@@ -1942,6 +1947,9 @@ RETRY_TRY_BLOCK:
                 while (c->eidx > ci->epos) {
                   ecall_adjust();
                 }
+                if (c->fib) {
+                  mrb_write_barrier(mrb, (struct RBasic*)c->fib);
+                }
                 mrb->c->status = MRB_FIBER_TERMINATED;
                 mrb->c = c->prev;
                 c->prev = NULL;