test: Zeroify scm buffer before filling

The CMSG_NXTHDR checks the length of the __next__ cmsg, i.e. the one
that it is about to return for filling. Thus, keeping there anything
but zeroes is unsafe.

Reported-by: Pavel Tikhomirov <snorcht@gmail.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>