valgrind doesn't know that struct sockaddr_rc contains some padding that
isn't used. Since we pass bind sizeof (struct sockaddr_rc) valgrind will
check all bytes are initialized. So explicitly clear all bytes before
passing to bind.

https://bugs.kde.org/show_bug.cgi?id=320116 tracks adding explicit
support to valgrind.

Change-Id: I05d2f221a5794228dae3077e8ea1bcda5ba1f8b0
Reviewed-on: https://gerrit.libreoffice.org/3998Reviewed-by: Fridrich Strba <fridrich@documentfoundation.org>
Tested-by: Fridrich Strba <fridrich@documentfoundation.org>