[videoinfoscanner] re-use fasthash if possible

No need to compute a slow hash in case the fast one already succeeded. Same already happens for shows.

@jmarshallnz for review please.
That looks OK from a quick scan, yeah.

I suspect, though, that the block below this bit can be cleaned up a bit?  You have about 3 if (bSkip) blocks atm, and duplicate `if (dbHash == hash)` checks.