Bug in OnlineMedian.java

[Gannys001]

At line 58, the new number should be pushed into lowerHalf when higherHalf is empty.

[chellitorvic]

But then it can result in NullPointerException at line 61 on the insertion of the next element, i.e. sequence [1, 2].

I've created pull request to address the issue #22

[bephrem1]

@chellitorvic Thanks, merged your pr