Fix maybe-uninitialized warning in Conv.h

Summary: Some compilers think result may be uninitialized

Reviewed By: francis-ma

Differential Revision: D4074526

fbshipit-source-id: 5df4568cfa5eb8f3b2993a82b831fd5e4028a281

folly/Conv.h

@@ -1490,7 +1490,7 @@ Expected<Tgt, detail::ParseToError<Tgt>> tryTo(StringPiece* src) {
 
 template <class Tgt>
 Tgt to(StringPiece* src) {
-  Tgt result;
+  Tgt result{};
   using Error = detail::ParseToError<Tgt>;
   return parseTo(*src, result)
       .thenOrThrow(