[MATH-1149] Fix potential null pointer dereferencing.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/math/trunk@1624752 13f79535-47bb-0310-9956-ffa450edef68
[MATH-1145] Fix integer overflows in MannWhitneyUTest. Thanks to Anders Conbere.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/math/trunk@1624756 13f79535-47bb-0310-9956-ffa450edef68