Return error if writer is nil

[boecklim]

Panics can happen in the function WriteMessageWithEncodingN()

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x18 pc=0xc22067]

goroutine 109 [running]:
github.com/libsv/go-p2p/wire.WriteMessageWithEncodingN({0x0, 0x0}, {0x27dfbc8, 0xc0224da7b8}, 0x8a27b8?, 0xe8f3e1e3, 0xce3ad94?)
	/home/runner/go/pkg/mod/github.com/libsv/[email protected]/wire/message.go:369 +0x747
github.com/libsv/go-p2p/wire.WriteMessageN(...)
	/home/runner/go/pkg/mod/github.com/libsv/[email protected]/wire/message.go:292
github.com/libsv/go-p2p/wire.WriteMessage({0x0?, 0x0?}, {0x27dfbc8?, 0xc0224da7b8?}, 0x540be400?, 0x2?)
	/home/runner/go/pkg/mod/github.com/libsv/[email protected]/wire/message.go:301 +0x2e
github.com/libsv/go-p2p.(*Peer).writeRetry.func1()

The panic may happen if the buffer hw is nil

[icellan]

@boecklim I don't think this is the solution. This would also mask the error to calling client.

hw is created just a few lines above, so I don't see how it is possible that it is nil. I think it is much more likely that the io.writer w is nil, which would mean this function is being incorrectly called. What you could do is check at the beginning of the function whether w is nil and return an error.