[x265] Completion of error handling

[SF Markus Elfring]

Hello,

I have looked at a few source files for your current software. I have noticed
that some checks for return codes are missing.

Would you like to add more error handling for return values from functions like
the following?
- signal ⇒ main
  https://bitbucket.org/multicoreware/x265/src/d11de33521cf/source/x265.cpp#cl-481

- pthread_join ⇒ Thread::stop

https://bitbucket.org/multicoreware/x265/src/d95d94ee0e1d/source/common/threading.cpp#cl-92

Regards,
Markus