Don't combine distribution if count == 0

Author: mitchellwrosen

cbits/distrib.c

@@ -29,6 +29,11 @@ void hs_distrib_add_n(struct distrib* distrib, StgDouble val, StgInt64 n) {
 // http://en.wikipedia.org/wiki/Algorithms_for_calculating_variance#Parallel_algorithm
 void hs_distrib_combine(struct distrib* b, struct distrib* a) {
   hs_lock(&b->lock);
+
+  if (!b->count) {
+    return;
+  }
+
   const StgInt64 count = a->count + b->count;
   const StgDouble delta = b->mean - a->mean;
   const StgDouble mean = (a->count * a->mean + b->count * b->mean) / count;