Remove an unnecessary null pointer check

[An extra null pointer check is not needed](https://stackoverflow.com/questions/18775608/free-a-null-pointer-anyway-or-check-first "Free a null pointer anyway or check first?") in [the function “va_DisplayContextDestroy”](https://github.com/intel/libva/blob/e4dc66b240d6d21e6546c9a21dbcfe455c8a6bff/va/glx/va_glx.c#L64-L67 "Update candidate").