fix for _get_by_slice on feature-less slices

Created by: danstaines

This was revealed as a problem when running densityfeature code (from GenomeLoader, not from the hive) on genomes with slices without features - it looks as if bind parameters were left hanging around in the adaptor instance as they weren't ever used (and hence cleared). This is probably the simplest fix.

Created by: coveralls

Coverage increased (+0.28%) when pulling 29430297 on danstaines:hotfix/get_by_slice into a1466ff6 on Ensembl:master.

Created by: magaliruffier

Looks good to me, thanks for the fix