COMP: fix parenthesis warning in debug mode

Fix warnings in debug mode that look like this:

/home/poughov/cnes/dev/otb/Modules/Learning/LearningBase/include/otbMachineLearningModel.hxx:150:58: warning: suggest parentheses around ‘&&’ within ‘||’ [-Wparentheses]
   assert((proba==nullptr)||(input->Size()==proba->Size())&&"Proba sample list and target label list do not have the same size.");