Assert that we can't construct centroid with weight <= 0

Summary:
We should not be able to create tdigest centroids with weight <= 0.
Added a debug assert to ensure this.

Differential Revision: D8786969

fbshipit-source-id: 738f2c8d00596365363ea7be88c759da881ab770

folly/stats/TDigest.h

@@ -51,7 +51,9 @@ class TDigest {
   class Centroid {
    public:
     explicit Centroid(double mean = 0.0, double weight = 1.0)
-        : mean_(mean), weight_(weight) {}
+        : mean_(mean), weight_(weight) {
+      DCHECK_GT(weight, 0);
+    }
 
     inline double mean() const {
       return mean_;