p2p/discover/v5wire: fix nil dereference on handshakeAuthData

[marcello33]

This PR fixes a nil pointer dereference in the decodeHandshakeMessage.
There's an improper handling of the auth variable returned by the decodeHandshake function when an error occurs.
The function decodeHandshakeMessage does not validate if auth is nil before dereferencing it.
Upstreaming this from Polygon PoS bor client PR.

Wrt the implementation, the check if auth != nil can be considered pleonastic next to auth.isHandshakeAuthDataValid(), but I decided to keep it to avoid static analysis tools (often integrated into IDEs) to scream about possible nil pointer dereferences, and would keep the code safe even when isHandshakeAuthDataValid is modified.

[fjl]

I don't fully understand your change. On master branch, the function decodeHandshakeAuthData does not return auth as a pointer, so it is not susceptible to nil dereference.

Your PR makes it return a pointer, and then you add a nil check to prevent the issue.

Can you please tell which issue you are actually trying to fix here?

[marcello33]

Right, I was looking at the accessed fields, but those are zero values and not nil. Thank you