Segfault when add mime as subpart to itself

[moteus]

I did this

curl_mime *mime = curl_mime_init(easy);
curl_mimepart *part = curl_mime_addpart(mime);
CURLcode a1 = curl_mime_subparts(part, mime);
curl_mime_free(mime);

I expected the following

curl_mime_free(mime); should not crash but curl_mime_subparts(part, mime); should returns CURLE_BAD_FUNCTION_ARGUMENT and do nothing.

curl/libcurl version

curl 7.56.0 (i386-pc-win32) libcurl/7.56.0 WinSSL
Release-Date: 2017-10-04
Protocols: dict file ftp ftps gopher http https imap imaps ldap pop3 pop3s rtsp smb smbs smtp smtps telnet tftp
Features: AsynchDNS Largefile SSPI Kerberos SPNEGO NTLM SSL

operating system

Windows 7 x64

[bagder]

I'm not convinced this is something we have to protect against, but it should be easy to at least prevent this bad use.

[bagder]

Let me know what you think of this fix!