Fix middleSplit_ for same points

[yzabalotski]

While #220 decreases amount of middleSplit_ calls, it also prevents finding correct min_elem and max_elem when max_span == 0 and span == 0. What leads to wrong cutval calculation and out of range reads at divideTree.

The issue can be observed with the unit test executed under valgrind.

[jlblancoc]

Good catch, thanks! 💯