radix rn_inithead: Fix minor leak in low memory conditions

R_Zalloc is essentially a malloc(M_NOWAIT) wrapper. It is possible that 'rnh'
failed to allocate, but 'rmh' succeeds. In that case, we bail out of
rn_inithead() but previously did not free 'rmh'.

Introduced in r287073 (projects/routing) / MFP r294706.

Reported by: Coverity
CID: 1350258
Sponsored by: EMC / Isilon Storage Division