TFTP: don't ack if wrong block num is received

If an unexpected block number was received, break out of the
switch loop.

lib/tftp.c

@@ -604,6 +604,7 @@ static CURLcode tftp_rx(tftp_state_data_t *state, tftp_event_t event)
               NEXT_BLOCKNUM(state->block));
         return CURLE_TFTP_ILLEGAL;
       }
+      break;
     }
     /* This is the expected block.  Reset counters and ACK it. */
     state->block = (unsigned short)rblock;