Use memmove in reassemble_rx_sdu UM since overlapping memory sometimes

lib/src/upper/rlc_um.cc

@@ -554,7 +554,7 @@ void rlc_um::reassemble_rx_sdus()
 
       log->debug("Concatenating %d bytes in to current length %d. rx_window remaining bytes=%d, vr_ur_in_rx_sdu=%d, vr_ur=%d, rx_mod=%d, last_mod=%d\n",
         len, rx_sdu->N_bytes, rx_window[vr_ur].buf->N_bytes, vr_ur_in_rx_sdu, vr_ur, cfg.rx_mod, (vr_ur_in_rx_sdu+1)%cfg.rx_mod);
-      memcpy(&rx_sdu->msg[rx_sdu->N_bytes], rx_window[vr_ur].buf->msg, len);
+      memmove(&rx_sdu->msg[rx_sdu->N_bytes], rx_window[vr_ur].buf->msg, len);
       rx_sdu->N_bytes += len;
       rx_window[vr_ur].buf->msg += len;
       rx_window[vr_ur].buf->N_bytes -= len;