Avoid panic when a netmap instance cannot obtain memory.

A uint32_t is always >= 0.

Sponsored by: ADARA Networks

sys/dev/netmap/netmap_mem2.c

@@ -325,7 +325,8 @@ netmap_new_bufs(struct netmap_if *nifp,
 	return;
 
 cleanup:
-	for (i--; i >= 0; i--) {
+	while (i > 0) {
+		i--;
 		netmap_obj_free(nm_mem->nm_buf_pool, slot[i].buf_idx);
 	}
 }