downloader: fix opening of non-existing files
downloader: fix opening of non-existing files

During os.walk() we might encounter symlinks that do not exist on the
host that calculates the hash. This happens when you generate an image
inside uchroot and calculate the hash outside of it.

We do not want to open those files for hashing.


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PolyJIT/benchbuild,PolyJIT/benchbuild,PolyJIT/benchbuild,PolyJIT/benchbuild