Fix leaked msgb introduced by the MGW support recently added, and from
there copied to the UPF support added after that.
Fixes leaked "RANAP Tx" msgb, one per RAB Assignment that involves an
MGW or UPF proxying of user data.
Related: SYS#6297
Change-Id: Ie30e880301346ffca72f98f8c467e56d622fb03f