hack in RLC AM to avoid a race

Here is the problem:
    UE comes. SRB2 is configured via message to RRC.
    At some point the RLC AM is created but not configured yet.
    At this moment (I think) MAC calls mac_rlc_status_ind
    which calls this function. But the init was not finished yet
    and we have a crash below when testing mem_block != NULL.

The "solution" is to test if rlc->input_sdus is NULL.
This is a very dirty hack. I would say the solution
is to use proper locking mechanism because RLC is used
by two threads: PHY/MAC on one hand and RRC on another
hand (I think).