[fib algo][dxr] Fix division by zero.

A division by zero would occur if DXR would be activated on a vnet
with no IP addresses configured on any interfaces.

PR: 257965
MFC after: 3 days
Reported by: Raul Munoz

(cherry picked from commit eb3148cc4d256c20b5c7c9052539139b6f57f58b)