Invalid HARQ PID at gNB in function nr_process_mac_pdu

Sometimes there is malformed packet in Msg3 that reaches MAC layer and enters in case UL_SCH_LCID_SINGLE_ENTRY_PHR: with HARQ PID -1 (RA).

The root cause is unknown, and the steps to reproduce it are also unclear. May be related with issue #572 (closed).

A workaround to keep gNB working, when this kind of packets reach MAC layer, was done in !1769 (merged)