drivers: ieee802154: nrf5: fix NULL pointer dereference
drivers: ieee802154: nrf5: fix NULL pointer dereference

When a frame is sent with a cleared ACK request bit, the transmit
metadata contains a NULL pointer to the ACK frame.

The pointer must not be dereferenced in such case.

Signed-off-by: Rafał Kuźnia <2e7ab041a15fd1b2ee150a94889a37d823e289e8@nordicsemi.no>

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