fix: prevent process crash when fileFilter calls its callback more than once (fixes #950)

[spokodev]

Fixes #950.

With upload.fields(), if a fileFilter invokes its callback more than once and does so asynchronously, Multer crashes the whole Node process with an uncaught exception:

TypeError: Cannot read properties of undefined (reading 'length')
    at FileAppender.removePlaceholder (lib/file-appender.js)

A placeholder is inserted synchronously when the file event fires, then removed asynchronously from the fileFilter callback. When the callback runs twice, removePlaceholder runs twice for the same field: the first call sees length 1 and deletes req.files[fieldname]; the second call reads .length of the now-missing entry and throws on a later tick, where there is no surrounding try/catch, so the process dies. This has been reported repeatedly (#950, #1093, #1205).

Only the OBJECT strategy (upload.fields()) is affected; the ARRAY strategy (upload.array()) already tolerates a double callback because arrayRemove no-ops on a missing entry.

Fix

Break out of the OBJECT branch when the field entry is already gone, matching the null-safety the ARRAY branch already has. The single-callback path and behavior are unchanged.

Testing

Added a test to test/file-filter.js: a .fields() parser whose fileFilter calls the callback twice via setImmediate (the asynchronous case). It crashes the process on the current code and passes with the fix. Full suite stays green (81 passing).