libcurl: security bump to version 7.57.0

Fixes the following security issues:

- CVE-2017-8816: NTLM buffer overflow via integer overflow
- CVE-2017-8817: FTP wildcard out of bounds read
- CVE-2017-8818: SSL out of buffer access

For more details, see the changelog:
https://curl.haxx.se/changes.html#7_57_0



Signed-off-by: Peter Korsgaard <peter@korsgaard.com>