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

sys/net/radix.c

@@ -1156,6 +1156,8 @@ rn_inithead(void **head, int off)
 	if (rnh == NULL || rmh == NULL) {
 		if (rnh != NULL)
 			R_Free(rnh);
+		if (rmh != NULL)
+			R_Free(rmh);
 		return (0);
 	}