The problem was in the Aria part of the range optimizer,
maria_records_in_range(), which wrong concluded that there was no rows
in the range.

This error would happen in the unlikely case when searching for a range
on a partial key and there was a match for the first key part in the
upper part of the b-tree (node) and also a match in the underlying
node page.

In other words, for this bug to happen one have to use Aria, have a multi
part key with a lot of identical values for the first key part and do a
range search on the second part of the key.

Fixed by ensuring that we do not stop searching for partial keys found
on node.

Other things:
- Added some comments
- Changed a variable name to more clearly explain it's purpose.
- Fixed wrong cast in _ma_record_pos() that could cause problems on 32 bit
  systems.