net: ipv6_fragment: fix NULL pointer dereference issues
net: ipv6_fragment: fix NULL pointer dereference issues

If we have less fragments than what can be stored in the reassembly
array, some loops will blindly dereference NULL pointers.

Add checks for NULL pointers when necessary and exit the loop.

Signed-off-by: Florian Vaussard <ef1e2bd37ca89620283a38ee0519838a257cab3b@gmail.com>

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