Check for a defined Slice object before asserting circularity tests.

A fix for the fix. Originally we started with passing through Slice
for the aligned feature but this is wrong. We need to assert the
hit feature Slice which we do not have. This change reverts the
_check_start_end_strand() method back to its original functionality.
Merge branch 'hotfix/basefeatureadaptor_circularitytest'