tests: bluetooth: tester: Fix NULL pointer dereference in error path
tests: bluetooth: tester: Fix NULL pointer dereference in error path

bt_conn_unref() requires valid conn pointer but could be called with
NULL in case valid connection was not found in disconnect_eatt_chans().

Fixes #39851

Signed-off-by: Szymon Janc <db186c297188f7acbbdc9bbd6aff2f45665bb42d@codecoup.pl>

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