Prevent panic in membership updater (#1021)

roomserver/internal/input_membership.go

@@ -16,6 +16,7 @@ package internal
 
 import (
 	"context"
+	"errors"
 	"fmt"
 
 	"github.com/matrix-org/dendrite/roomserver/api"
@@ -106,6 +107,13 @@ func updateMembership(
 		return updates, nil
 	}
 
+	if add == nil {
+		// This shouldn't happen. Returning an error here is better than panicking
+		// in the membership updater functions later on.
+		// TODO: Why does this happen to begin with?
+		return updates, errors.New("add should not be nil")
+	}
+
 	mu, err := updater.MembershipUpdater(targetUserNID)
 	if err != nil {
 		return nil, err