crosscluster/physical: defensively check errCh in span config event s…

…tream

Select does not choose which channel to read from based on some order, but the
span config event stream must return as soon as there is an error. For this
reason, this patch rechecks the errCh after select chooses to read from the
data channel.

Informs #128865

Release note: none

pkg/ccl/crosscluster/producer/span_config_event_stream.go

@@ -157,7 +157,12 @@ func (s *spanConfigEventStream) Next(ctx context.Context) (bool, error) {
 	case err := <-s.errCh:
 		return false, err
 	case s.data = <-s.streamCh:
-		return true, nil
+		select {
+		case err := <-s.errCh:
+			return false, err
+		default:
+			return true, nil
+		}
 	}
 }