fix for potential stack overflow...

... which frequently occured when running unit tests with the -DrunSlowTests=true option,

stack trace:
...
at com.tdunning.math.stats.AVLTreeDigest.add(AVLTreeDigest.java:135)
at com.tdunning.math.stats.AVLTreeDigest.compress(AVLTreeDigest.java:165)
at com.tdunning.math.stats.AVLTreeDigest.add(AVLTreeDigest.java:135)
at com.tdunning.math.stats.AVLTreeDigest.compress(AVLTreeDigest.java:165)
at com.tdunning.math.stats.AVLTreeDigest.add(AVLTreeDigest.java:135)
at com.tdunning.math.stats.AVLTreeDigest.compress(AVLTreeDigest.java:165)