backlight: Prevent dereference of potential NULL argv

backlight: Prevent dereference of potential NULL argv

Adam Sampson spotted that

"It's possible (but not very sensible) to exec a program with an empty
argument list, so argv[0] is not necessarily a valid pointer. For
example:

$ cat exec0.c

int main(int argc, char *argv[]) {
    char *empty[1] = { NULL };
    execvp(argv[1], empty);
    perror("execvp");
    return 1;
}
$ ./exec0 /usr/libexec/xf86-video-intel-backlight-helper
Usage: (null) <iface>
"

He sensibly suggested that we hardcode the program name to avoid the
NULL dereference. Being the paranoid type, we should also be careful not
to write to any file descriptors outside of our control (i.e. stderr),
so disable the messages unless we are debugging.

Reported-by: Adam Sampson
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>