Bluetooth: Mesh: Fix unref null pointer
Bluetooth: Mesh: Fix unref null pointer

When pb-gatt advertising enabled, after extablish connect,
will call `cb->connected` and `cb->adv_send`.

In previous connected also clear `ADV_FLAG_PROXY` flag, but
in `adv_send` will attempt unref null point buffers.

Signed-off-by: Lingao Meng <d107961e9e4fef34975d9e927886461599dc7eda@xiaomi.com>

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