Blender Git Loki

VSE: handle IO errors when reading disk cache header and version

Respond to return values of `fscanf()` and `fread()` to detect and
handle I/O errors. Not only is error handling a good idea, this also
prevents warnings from GCC that `fread()` and `fscanf()` return values
are ignored.

Reviewed By: ISS

Differential Revision: https://developer.blender.org/D9915