[x265] Fix signal handler

[SF Markus Elfring]

> The sigint handler is only for early-exiting the encode loop; all of the
> exit() calls would happen before the encode loop is entered, so I don't think
> this is a problem.

I prefer to avoid such an open issue. I suggest to make the affected
implementation details async-signal-safe.

Would you like to use the function "_exit" instead?
http://pubs.opengroup.org/onlinepubs/9699919799/functions/_exit.html

Regards,
Markus