add check for invalid nof subheaders in MAC PDU

lib/src/common/pdu.cc

@@ -133,6 +133,11 @@ uint8_t* sch_pdu::write_packet(srslte::log *log_h)
     return NULL; 
   }
 
+  if (nof_subheaders <= 0) {
+    log_h->error("Trying to write packet with invalid number of subheaders (nof_subheaders=%d).\n", nof_subheaders);
+    return NULL;
+  }
+
   /* If last SDU has zero payload, remove it. FIXME: Why happens this?? */
   if (subheaders[nof_subheaders-1].get_payload_size() == 0) {
     del_subh();