From Axel: Fix error calculation for THnSparse: first calculate the bin's current error, then add new bin content. Also remove a few stray "return"s.
From Axel:
Fix error calculation for THnSparse: first calculate the bin's current
error, then add new bin content. Also remove a few stray "return"s.


git-svn-id: ecbadac9c76e8cf640a0bca86f6bd796c98521e3@21560 27541ba8-7e3a-0410-8455-c3a389f83636

dawehner/root,bbannier/ROOT,bbannier/ROOT,bbannier/ROOT,bbannier/ROOT,bbannier/ROOT,bbannier/ROOT,bbannier/ROOT,dawehner/root,dawehner/root,dawehner/root,dawehner/root,dawehner/root,dawehner/root,dawehner/root