Guard createOrUpdateDevice() in gatewayBT.cpp against null or >17-char MAC
strings before the strcpy into the 18-byte BLEdevice::macAdr buffer. An
MQTT client could previously publish a crafted white-list/black-list/id
field to MQTTtoBT/config (and related topics) and corrupt the heap.
Apply the same defense to the XtoSYS config branch in main.cpp: mqtt_user,
mqtt_pass, mqtt_server, and mqtt_port are now length-checked against their
destination buffer sizes before strcpy. The mqtt_port destination is only
6 bytes, so this path was particularly easy to overflow via MQTTtoSYS/config.
Oversized fields are now rejected with a warning log; existing values are
preserved and the device does not crash.
Co-authored-by: Florian <1technophile@users.noreply.github.com>
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
