cancel RA when receiving contention resolution in wrong state

this fixes a bug in which the contention resolution timer expired
and the RA proc entered the BACKOFF state.
In the same TTI, a MAC PDU with the contention reslution ID is received
and the already aborted RA procedure succeeds. Unfortunatly with the
wront c-rnti which has already been reset before. See log excerpt
below:

16:16:36.909687 [RRC ] [D] MEAS:  Processing measurement of 1 cells
16:16:36.909706 [RRC ] [I] MEAS:  New measurement serving cell: rsrp=-85.00 dBm.
16:16:36.909732 [MAC ] [I] [ 6283] RA:    ConRes: Contention Resolution Timer expired. Stopping PDCCH Search and going to Response Error
16:16:36.909749 [MAC ] [D] [ 6283] RA:    ConRes: Backoff wait interval 143
16:16:36.909771 [MAC ] [D] [ 6282] Delivering PDU=27 bytes to Dissassemble and Demux unit (Temporal C-RNTI)
16:16:36.909799 [MAC ] [D] [ 6283] Found Contention Resolution ID CE
16:16:36.909814 [MAC ] [D] [ 6283] RA:    Backof: MAC PDU Contains Contention Resolution ID CE
16:16:36.909826 [MAC ] [D] [ 6283] Msg3 buffer flushed
16:16:36.909858 [MAC ] [I] [ 6283] RA:    Backof: Random Access Complete.     c-rnti=0x0, ta=0
16:16:36.909871 [MAC ] [D] [ 6283] Saved MAC PDU with Temporal C-RNTI in buffer

srsue/src/stack/mac/proc_ra.cc

@@ -572,6 +572,11 @@ bool ra_proc::contention_resolution_id_received(uint64_t rx_contention_id)
 
   rDebug("MAC PDU Contains Contention Resolution ID CE\n");
 
+  if (state != CONTENTION_RESOLUTION) {
+    rError("Received contention resolution in wrong state. Aborting.\n");
+    response_error();
+  }
+
   // MAC PDU successfully decoded and contains MAC CE contention Id
   contention_resolution_timer.stop();