handle edge case of symlink to something that is not really a pointer file
That seems very unlikely to happen, but still, it's possible it could.
And with the recent addition of locked files to the keys db, this could
be called by places that did not call it before, so it seems even more
important it's correct.
Adds an extra stat of the file, and is potentially racy, but both
problems are fixed by the unix-2.8.0 path. I have not tested that path
builds because that package is not yet released and it would be difficult
to install it since it's tightly tied to a ghc version.