Bluetooth: gatt: Fix notification cleanup on disconnection
Bluetooth: gatt: Fix notification cleanup on disconnection

This fixes GATT notification buffer missing cleanup on ACL
disconnection. Fixes potential NULL pointer dereference trying to use
invalid conn object in `notify_mult_process` work queue task handler
and/or missing free of pending notification buffer.

Signed-off-by: Mariusz Skamra <c73850bd0bf85e95dce44c2ce1418ca55c1b6c5d@codecoup.pl>

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