musl: add fwrite regression patch

As pointed in http://www.openwall.com/lists/musl/2016/02/17/3

 add the
fwrite regression patch for the recent 1.1.13 release.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>