bump nim-eth and nim-nat-traversal

[tersec]

From nim-eth:

• handle new UPnP return value nim-eth#728

From nim-nat-traversal:

• fix/update UPNP_GetValidIGD() return value handling nim-nat-traversal#29

Avoids a potential crash while setting up UPnP.

[etan-status]

Thanks. Good catch, and sustainable fix, that we did not handle unexpected output codes gracefully.
Have smoke-tested locally but IGD worked fine so didn't hit it.