fix: prevent file handle leak when maxFiles is exceeded

[guoyangzhen]

Fixes #987

Problem

When options.maxFiles is set and a request exceeds the limit, file handles are leaked and never released until the process exits.

Root cause in _handlePart:

1. this.emit('fileBegin', part.name, file) — triggers the maxFiles check
2. maxFiles check calls this._error(), destroying files in this.openedFiles
3. But _handlePart continues and calls file.open() + this.openedFiles.push(file)
4. This new file stream is opened after the error handling ran, so it's never closed

Fix

Check this.error after emitting fileBegin but before file.open(). If an error occurred (e.g., maxFiles exceeded), decrement _flushing and return early to prevent opening the file stream.

Reproduction

import formidable from 'formidable'

const form = formidable({
  maxFiles: 1,
  uploadDir: './uploads',
  keepExtensions: true,
})

// Upload 2 files — the 2nd file's write stream is never closed
form.parse(req)

With the fix, no file handles are leaked when maxFiles is exceeded.