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>

test/zdtm/static/scm03.c

@@ -22,6 +22,7 @@ static int send_fd(int via, int fd1, int fd2)
 	char c = '\0';
 	int *fdp;
 
+	memset(buf, 0, sizeof(buf));
 	h.msg_control = buf;
 	h.msg_controllen = sizeof(buf);
 #ifdef SEPARATE