Reset size of byte slice from buffer pool to 0 for Distinct scans

The buffer used for distinct comparison needs to be initialized to a zero length slice after checking out from buffer pool. Failure observed in 

http://ci2i-unstable.northscale.in/gsi-plasma-27.02.2017-11.42.fail.html

http://ci2i-unstable.northscale.in/gsi-plasma-26.02.2017-11.41.fail.html

Logs and test output both can be found in above links.