Improve support for lrange negative start indices

If the start index is negative and less than `-list.size`, nil was
being returned. This does not mirror real Redis behaviour.

See spec changes for an example.

Change-Id: Ifeb73d72e58924d7af209ff7ca045c81c862e4f3
Reviewed-on: https://gerrit.causes.com/33214
Reviewed-by: Shane da Silva <shane@causes.com>
Tested-by: Shane da Silva <shane@causes.com>