quiet -Wmaybe-uninitialized

Summary: gcc-4.9 doesn't appreciate this clever loop and worries about `prev` being uninitialized on first use. Assuming the rest of the program behaves properly and preserves the invariant this code is expecting, this isn't bugged. But let's initialize it anyway.

Reviewed By: meyering

Differential Revision: D4120746

fbshipit-source-id: 1a7c323a4b6296a81768a62dd7d72337c5c35bf0

folly/stats/MultiLevelTimeSeries-defs.h

@@ -50,7 +50,7 @@ MultiLevelTimeSeries<VT, CT>::MultiLevelTimeSeries(
 
   levels_.reserve(durations.size());
   int i = 0;
-  Duration prev;
+  Duration prev{0};
   for (auto dur : durations) {
     if (dur == Duration(0)) {
       CHECK_EQ(i, durations.size() - 1);