Bluetooth: Mesh: Null check buf before unref
Bluetooth: Mesh: Null check buf before unref

The frnd->last buffer can potentially be NULL if friend_clear is called
after the adv.c buf->busy check, but before the adv_start callback.
The current design is based on the adv.c thread being cooperative, and
therefore not yielding between the busy check and the adv_start
callback, but as the bt_le_adv_start call has to acquire a semaphore,
there's a possibility for friend_clear being called inbetween.

Fixes #26177.

Signed-off-by: Trond Einar Snekvik <cbf21a7cf058d7ce8e74b0726695057d761f6ae9@nordicsemi.no>

galak/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,Vudentz/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,Vudentz/zephyr,Vudentz/zephyr,Vudentz/zephyr,galak/zephyr,Vudentz/zephyr,finikorg/zephyr,galak/zephyr,nashif/zephyr,galak/zephyr,Vudentz/zephyr