libcurl: security bump version to 7.50.1

Contains fixes for CVE-2016-5419..5421:
https://curl.haxx.se/docs/vuln-7.50.0.html



[Peter: extend commit message with CVE info]
Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>