net: route: Do not dereference NULL pointer while getting next hop
net: route: Do not dereference NULL pointer while getting next hop

Assertions should only be used to check invariants.  Things that may
change value in runtime are better left to proper checks.

Signed-off-by: Leandro Pereira <51c7437a02b34df685e6b4ecb2e9bca23efe2e1a@intel.com>

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