fix: prevent file handle leak when maxFiles is exceeded

Fixes #987

When maxFiles limit is reached, the fileBegin event handler calls _error(),
but _handlePart continues and opens a write stream for the new file.
These file handles are never closed.

Fix: check this.error after emitting fileBegin and before file.open().
If an error occurred (e.g., maxFiles exceeded), decrement _flushing
and return early to prevent the file stream from being opened.

Author: guoyangzhen

src/Formidable.js

@@ -403,6 +403,12 @@ class IncomingForm extends EventEmitter {
     });
     this.emit('fileBegin', part.name, file);
 
+    // Check for error after fileBegin (e.g., maxFiles exceeded) to avoid leaking file handles
+    if (this.error) {
+      this._flushing -= 1;
+      return;
+    }
+
     file.open();
     this.openedFiles.push(file);