I'd recommend updating this comment accordingly.

The rtnh_weight is defined as unsigned. Certainly a negative weight does not make sense.

struct rta_mpath_nh {
        struct sockaddr *gw;
        uint32_t        ifindex;
        uint8_t         rtnh_flags;
        uint8_t         rtnh_weight;
        uint32_t        rtax_mtu;
        uint32_t        rta_rtflags;
        uint32_t        rta_expire;
};

If weight is 0, it's undefined.
But you're right, it's impossible now to be 0 since we overwrite it to RT_DEFAULT_WEIGHT anyway.
I've done it right in D55506, as time passes I tend to forgot :)