DecisionTree warnings with gcc

[Chris Beall]

In file included from /home/cbeall3/git/gtsam/gtsam/discrete/AlgebraicDecisionTree.h:21:0,
                 from /home/cbeall3/git/gtsam/gtsam/discrete/Potentials.h:20,
                 from /home/cbeall3/git/gtsam/gtsam/discrete/DecisionTreeFactor.h:22,
                 from /home/cbeall3/git/gtsam/gtsam_unstable/discrete/BinaryAllDiff.h:12,
                 from /home/cbeall3/git/gtsam/gtsam_unstable/discrete/AllDiff.h:10,
                 from /home/cbeall3/git/gtsam/gtsam_unstable/discrete/AllDiff.cpp:9:
/home/cbeall3/git/gtsam/gtsam/discrete/DecisionTree-inl.h: In member function ‘gtsam::DecisionTree<L, Y>::NodePtr gtsam::DecisionTree<L, Y>::compose(Iterator, Iterator, const L&) const [with Iterator = __gnu_cxx::__normal_iterator<gtsam::DecisionTree<long unsigned int, double>*, std::vector<gtsam::DecisionTree<long unsigned int, double>, std::allocator<gtsam::DecisionTree<long unsigned int, double> > > >; L = long unsigned int; Y = double; gtsam::DecisionTree<L, Y>::NodePtr = boost::shared_ptr<const gtsam::DecisionTree<long unsigned int, double>::Node>]’:
/home/cbeall3/git/gtsam/gtsam/discrete/DecisionTree-inl.h:489:89: warning: ‘*((void*)& nrChoices +8)’ may be used uninitialized in this function [-Wmaybe-uninitialized]
     boost::shared_ptr<Choice> choiceOnHighestLabel(new Choice(*highestLabel, *nrChoices));
                                                                                         ^