The default error handler `wp_handle_upload_error()` expects a reference for the first parameter but `call_user_func()` doesn't pass parameters by reference. The current code didn't produce any issues until now. PHP 7.0.9 (and PHP 7.1) is now stricter and prevents calling the error handler with a warning:
> PHP Warning: Parameter 1 to wp_handle_upload_error() expected to be a reference, value given.
To restore the error handler `_wp_handle_upload()` now uses `call_user_func_array()`.
Props jbrinley.
Props jorbin for review.
Fixes#37570.
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