Fix a bug in RegionGrowing with oversized output cluster vector.

The clusters vector output by RegionGrowing<PointT, NormalT>::extract()
function was not properly trimmed after filtering out clusters that are
too small and too large. As a result many dozens/hundreds of zero-sized
clusters seeped into the output vector.
Merge pull request #205 from taketwo/region-growing-fix

Fix a bug in RegionGrowing with oversized output cluster vector.