mac: make sure we only pull out valid rapid from subheader

srsue/src/stack/mac/proc_ra.cc

@@ -326,7 +326,7 @@ void ra_proc::tb_decoded_ok() {
   current_ta = 0; 
   
   while(rar_pdu_msg.next()) {
-    if (rar_pdu_msg.get()->get_rapid() == sel_preamble) {
+    if (rar_pdu_msg.get()->has_rapid() && rar_pdu_msg.get()->get_rapid() == sel_preamble) {
 
       rar_received = true; 
       process_timeadv_cmd(rar_pdu_msg.get()->get_ta_cmd());
@@ -377,10 +377,12 @@ void ra_proc::tb_decoded_ok() {
         contention_resolution_timer->run();
       }
     } else {
+      if (rar_pdu_msg.get()->has_rapid()) {
         rInfo("Found RAR for preamble %d\n", rar_pdu_msg.get()->get_rapid());
       }
     }
   }
+}
 
 void ra_proc::step_response_reception(uint32_t tti)
 {