net: ip: ipv6_nbr: Fix uninitialized variable in ipv6_nbr
net: ip: ipv6_nbr: Fix uninitialized variable in ipv6_nbr

Setting it to UINT32_MAX, as it is subsequently overwritten with
MIN(oldest, something_else).

Signed-off-by: Piotr Zierhoffer <e1d19b12230bfc9d1ee7dc1e438ebc847bc06dad@antmicro.com>

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