- protect against  wrong input to acos due to numerical errors was 
 - remove assignement u[2]=u[1] as in CERNLIB routine. The gsl cubic solver is used and therefore a different order of the roots is returned. 

This fixes the bug reported in https://savannah.cern.ch/bugs/?49031


git-svn-id: http://root.cern.ch/svn/root/trunk@28158 27541ba8-7e3a-0410-8455-c3a389f83636