CVE-2026-15891 NULL pointer dereference in Zephyr MQTT-SN client when removing a non-responsive gateway

The MQTT-SN client keepalive handler processping in subsys/net/lib/mqttsn/mqttsn.c removes the gateway record after PINGREQ retries are exhausted. It invoked SYSSLISTPEEKHEADCONTAINER&client-gateways, gw, next but discarded the result. That macro is a pure expression that does not assign to gw, s...