seccomp_user_notif.2: EXAMPLE: correct the check for NUL in buffer returned by read()

seccomp_user_notif.2: EXAMPLE: correct the check for NUL in buffer returned by read()

In the usual case, read(fd, buf, PATH_MAX) will return PATH_MAX
bytes that include trailing garbage after the pathname. So the
right check is to scan from the start of the buffer to see if
there's a NUL, and error if there is not.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>