In the TVectorT, TMatrixT free Add functions, add the special case

where the argument scalar is null, ie
 -if scalar != 0 target += scalar * A * source
 -if scalar == 0 target = A * source


git-svn-id: http://root.cern.ch/svn/root/trunk@14482 27541ba8-7e3a-0410-8455-c3a389f83636