fix: add nil check before writing to dest in handshake handler

[abs2023]

Summary

Fixes secondary crash discovered after PR #83 deployment. Adds defensive nil checks before attempting to write to destination during handshake phase.

Root Cause

After deploying PR #83 (which fixed the logging crash), a second nil pointer issue was discovered at 2026-02-10 20:56 UTC:

When miners send unexpected protocol messages before the destination connection is initialized (e.g., eth_submitLogin from Ethereum miners connecting to Bitcoin proxy), the code attempts to forward the message via p.proxy.dest.Write(). However, p.proxy.dest is nil during early handshake, causing panic at conn_dest.go:139 via handler_first_connect.go:83.

Changes Made

Added defensive nil checks in handler_first_connect.go before writing to dest:

1. MiningExtranonceSubscribe case:

   • Check if p.proxy.dest != nil before forwarding
   • Return nil, nil if dest not ready (safe drop)

2. Default case (unknown messages):

   • Check if p.proxy.dest != nil before forwarding
   • Return nil, nil if dest not ready (safe drop)
   • Updated comment to clarify behavior

Impact

• ✅ Resolves secondary crash pattern at conn_dest.go:139
• ✅ Handles protocol mismatches gracefully (e.g., Ethereum miners on Bitcoin proxy)
• ✅ Complements PR fix: handle mining.extranonce.subscribe and prevent nil pointer crash #83 to fully stabilize handshake phase
• ✅ No performance impact (just pointer checks)

Evidence

Crash pattern:

panic: runtime error: invalid memory address or nil pointer dereference  
[signal SIGSEGV: segmentation violation code=0x1 addr=0x110 pc=0x8feae6]
at conn_dest.go:139 via handler_first_connect.go:83

Trigger log (showing PR #83 logging fix working):

{"level":"warn","logger":"PRX","msg":"unknown handshake message from source: {\"id\":2,\"method\":\"eth_submitLogin\"...}",
"SrcAddr":"188.166.37.58:47284","DstAddr":"not-initialized","DstPort":"not-connected"}

The logging successfully showed "not-initialized" and "not-connected" (PR #83 fix working), but then crashed trying to forward the message.

Dependencies

• Requires PR fix: handle mining.extranonce.subscribe and prevent nil pointer crash #83 to be merged first (already done)
• This is a follow-up fix discovered during production monitoring

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