tbf_dl: correct tbf name in log message for moving a DL TBF

It makes no sense to call functions on null pointer object. Use
the name of the old tbf.

Change-Id: I93b8c07a0b2de40a11e94fd6c212897cbe3b50ef
Reviewed-on: https://gerrit.osmocom.org/212
Tested-by: Jenkins Builder
Reviewed-by: Holger Freyther <holger@freyther.de>
2 files changed