survive disconnect during BLE MTU lookup

main/trikke_ble_transport.c

@@ -340,6 +340,12 @@ static trikke_transport_status_t ble_poll_once(void *context)
     portEXIT_CRITICAL(&ble->lock);
 
     const uint16_t mtu = ble_att_mtu(connection_handle);
+    if (mtu == 0) {
+        // The connection can disappear after the locked state snapshot above.
+        // The GAP callback advances the delivery epoch; retain ownership and
+        // let a later poll restart the frame after resubscription.
+        return TRIKKE_TRANSPORT_PENDING;
+    }
     if (mtu <= 3 + TRIKKE_BLE_FRAGMENT_HEADER_SIZE) {
         return TRIKKE_TRANSPORT_FATAL;
     }