net: route: Check that neighbor exists before deleting it
net: route: Check that neighbor exists before deleting it

If the neighbour does not exists, then the route to it cannot
be deleted so we can return error to caller in that case.

Coverity-CID: 188173
Fixes #10090

Signed-off-by: Jukka Rissanen <f9f077d1da4c7aa947f9864fa5a28c3588f6a45d@linux.intel.com>

punitvara/zephyr,punitvara/zephyr,galak/zephyr,Vudentz/zephyr,nashif/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,galak/zephyr,kraj/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,Vudentz/zephyr,kraj/zephyr,explora26/zephyr,kraj/zephyr,explora26/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,ldts/zephyr,ldts/zephyr,finikorg/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr,kraj/zephyr,finikorg/zephyr,finikorg/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,finikorg/zephyr,Vudentz/zephyr,nashif/zephyr,Vudentz/zephyr,ldts/zephyr,explora26/zephyr,kraj/zephyr,nashif/zephyr