Avoid loosing DL tbf from old_ms when rx premier UL data

Scenario:
MS has UL TBF to attach to the network in FINISHED state, PCU received
DL data to continue with registration and creates a DL TBF in ASSIGN
state (through PCH).
At that point, the MS creates a new UL TBF through RACH request and PCU
creates it with a new MS object (because TLLI is not known at that
time).
When first UL data is received, due to contention resolution the PCU
learns the TLLI and finds out it already has another older MS object
with that TLLI, hence a duplicate and a merge must be done.

During that merge of the 2 MS objects, the DL TBF in ASSIGN state was
being freed. Let's actually not free it since we'll need to tell that DL
TBF in this scenario that it should instead re-start assignment through
PACCH.
Otherwise the MS keeps trying to register but the PCU is never able to
establish a DL TBF because it keeps assigning it through PCH instead of
PACCH.

Related: OS#5700
Change-Id: I9a989af89f9469d7894b9e70a2ea01989b9dbb75
---
M src/gprs_ms.c
M src/tbf.cpp
M src/tbf_dl.cpp
M tests/tbf/TbfTest.cpp
M tests/tbf/TbfTest.err
5 files changed, 44 insertions(+), 46 deletions(-)