[x265] Remove unnecessary check in destructors

[SF Markus Elfring]

Hello,

The null pointer check for the pointer "buf" is not needed in destructors for
classes like "Y4MOutput" and "YUVOutput".
http://dietmar-kuehl.de/mirror/c++-faq/freestore-mgmt.html#faq-16.8

https://bitbucket.org/multicoreware/x265/src/1d710404b8bb/source/output/y4m.cpp#cl-44
https://bitbucket.org/multicoreware/x265/src/1d710404b8bb/source/output/yuv.cpp#cl-40

Regards,
Markus