For that, use the same logic as TryBoxNmToPtr
Regression from 0c008ab081aa5bbf53f8562e95e547deae5afc2e
"tdf#157132: restore behaviour for TBL_RELBOXNAME"
Change-Id: I8f93ad3a9686001fd6bd7226400925e43b81ec23
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/159865
Tested-by: Jenkins
Reviewed-by: Xisco Fauli <xiscofauli@libreoffice.org>