Reset RX signal state at start of each receive window

src/lora_transport.cpp

@@ -202,6 +202,9 @@ bool lora_receive_window(LoraPacket &pkt, uint32_t timeout_ms) {
   if (timeout_ms == 0) {
     return false;
   }
+  g_last_rx_signal_valid = false;
+  g_last_rx_rssi_dbm = 0;
+  g_last_rx_snr_db = 0.0f;
   LoRa.receive();
   bool got = lora_receive(pkt, timeout_ms);
   LoRa.sleep();