curl: update to version 7.51.0

curl: update to version 7.51.0

This fixes the following security problems:
CVE-2016-8615: cookie injection for other servers
CVE-2016-8616: case insensitive password comparison
CVE-2016-8617: OOB write via unchecked multiplication
CVE-2016-8618: double-free in curl_maprintf
CVE-2016-8619: double-free in krb5 code
CVE-2016-8620: glob parser write/read out of bounds
CVE-2016-8621: curl_getdate read out of bounds
CVE-2016-8622: URL unescape heap overflow via integer truncation
CVE-2016-8623: Use-after-free via shared cookies
CVE-2016-8624: invalid URL parsing with '#'
CVE-2016-8625: IDNA 2003 makes curl use wrong host

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>