curl_formfree(3) on macOS

The text 87b2afa62a5e, as macOS 10.12.5, macOS 10.12.4, macOS 10.12.3, macOS 10.12.2, macOS 10.12.1, macOS 10.12 shipped it.

free a previously build multipart/formdata HTTP POST chain

curl_formfree(3) on macOS now