fib_dxr: do not leak memory if FIB constellation hits structural limit

DXR lookup table encoding has an inherent structural limit on the amount
of binary search ranges it can accomodate. With the current IPv4 BGP views
(circa 1 M prefixes) and default DXR encoding we are only at around 5% of
that limit, so far, far away from hitting it. Just in case it ever gets
hit, make sure we free the allocated structures, instead of leaking it.

MFC after: 1 week