Don't call mac_rlc_data_ind when SRB0 already exist for UE.

It can happen that L1 gives us a TB that we interpret as receiving UL
MAC TB with LCID "UL_SCH_LCID_CCCH1". This might or might not be
correct, but currently, we observe segfaults when trying to encode the
cellgroup in send_initial_ul_rrc_message() (encode_cellGroupConfig()).

This fix prevents a segfault by not delivering a MAC SDU to RLC if CCCH
is already present. Note that in some cases, this could still happen,
e.g. Resume Request. However, we don't support this message, so ignore
it for the moment.

This fix enables a gNB to operate continuously for extended periods
(days), which is essential for real deployment scenarios.

Closes: #756