[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.
This is looking much nicer/more obvious, but I think we can also split this one.  i.e.
```
if (hash == dbHash)
{
  // skipping - yay!
  bSkip = true;
}
else if (hash.empty())
{
  // directory empty or non-existent -add to clean list and skip
  bSkip = true;
}
else if (dbHash.empty())
{ // new folder
}
else
{ // changed, so rescan
}