Prevent accessing invalidated iterators in TDigest::merge

Summary:
According to the standard, `std::vector::insert` invalidates the end iterator
(and any iterators past the point of insertion). This seems to be benign in
practice but MSVC's iterator debug assertions catch it. We can preserve the spirit
of the original code by replacing the iterators with pointers or indexes. I chose
the latter.

Differential Revision: D47218022

fbshipit-source-id: 96808b20a9483b0ce0a4939edb3de48f48ef4f22

Author: Deniz Evrenci

third-party/folly/src/folly/stats/TDigest.cpp

@@ -222,7 +222,7 @@ TDigest TDigest::merge(Range<const TDigest*> digests) {
   std::vector<Centroid> centroids;
   centroids.reserve(nCentroids);
 
-  std::vector<std::vector<Centroid>::iterator> starts;
+  std::vector<size_t> starts;
   starts.reserve(digests.size());
 
   double count = 0;
@@ -233,7 +233,7 @@ TDigest TDigest::merge(Range<const TDigest*> digests) {
   double max = -std::numeric_limits<double>::infinity();
 
   for (const auto& digest : digests) {
-    starts.push_back(centroids.end());
+    starts.push_back(centroids.size());
     double curCount = digest.count();
     if (curCount > 0) {
       DCHECK(!std::isnan(digest.min_));
@@ -262,9 +262,12 @@ TDigest TDigest::merge(Range<const TDigest*> digests) {
         // digestsPerBlock big). In that case, merge to end. Otherwise, merge to
         // the end of that block.
         auto last = (i + (digestsPerBlock * 2) < startsSize)
-            ? *(starts.begin() + i + 2 * digestsPerBlock)
-            : centroids.end();
-        std::inplace_merge(first, middle, last);
+            ? starts[i + 2 * digestsPerBlock]
+            : centroids.size();
+        std::inplace_merge(
+            centroids.begin() + first,
+            centroids.begin() + middle,
+            centroids.begin() + last);
       }
     }
   }