Fix Busboy teardown race and multi-error handling during multipart parsing

[mukeshdhadhariya]

Summary

This PR addresses a teardown timing issue #1389 in make-middleware.js that affects Busboy error handling.

Two related problems are observed:

1. Busboy may emit a late error after teardown begins.
2. In malformed multipart flows, Busboy can emit multiple errors, causing:
   • uncaught Unexpected end of form
   • done() being called multiple times

Root Cause

Listener cleanup timing interacts with Busboy's error emission order.
Using process.nextTick alone does not fully guarantee safe handling when multiple errors are emitted.

Evidence

Failing test:

npx mocha --reporter spec --exit --check-leaks express-integration.js --grep "two errors to be emitted by busboy"

Observed failures:

• Uncaught Error: Unexpected end of form
• Error: done() called multiple times

Fix

This change ensures teardown and error handling remain consistent by preventing duplicate completion and ensuring late Busboy errors are safely absorbed.

[jonchurch]

slop