Fix SIGSEV in test

Signed-off-by: Eduardo J. Ortega U. <5791035+ejortegau@users.noreply.github.com>

go/vt/vtctl/reparentutil/replication.go

@@ -239,8 +239,18 @@ func stopReplicationAndBuildStatusMaps(
 				err = vterrors.Wrapf(err, "error when getting replication status for alias %v: %v", alias, err)
 			}
 		} else {
+			isTakingBackup := false
+
+			// Prefer the most up-to-date information regarding whether the tablet is taking a backup from the After
+			// replication status, but fall back to the Before status if After is nil.
+			if stopReplicationStatus.After != nil {
+				isTakingBackup = stopReplicationStatus.After.BackupRunning
+			} else if stopReplicationStatus.Before != nil {
+				isTakingBackup = stopReplicationStatus.Before.BackupRunning
+			}
+
 			m.Lock()
-			res.tabletsBackupState[alias] = stopReplicationStatus.After.BackupRunning
+			res.tabletsBackupState[alias] = isTakingBackup
 			m.Unlock()
 
 			var sqlThreadRunning bool