Merge pull request #4451 from crazy-max/fix-panic-history

llbsolver: fix possible panic when setting event to nil

solver/llbsolver/history.go

@@ -819,7 +819,7 @@ func (h *HistoryQueue) Listen(ctx context.Context, req *controlapi.BuildHistoryR
 		}
 		h.mu.Unlock()
 		for _, e := range events {
-			if e.Record == nil {
+			if e == nil || e.Record == nil {
 				continue
 			}
 			if err := f(e); err != nil {