fix collect for non-default-constructible types

Summary: as above

Test Plan: compile

Reviewed By: agoder@fb.com

Subscribers: folly-diffs@, jsedgwick, yfeldblum, chalfant

FB internal diff: D1999218

Signature: t1:1999218:1429217794:0278955bb471fe73a14ccedb59eae80b98acdbf3

folly/futures/Future-inl.h

@@ -586,9 +586,8 @@ namespace detail {
 
 template <typename T>
 struct CollectContext {
-  explicit CollectContext(int n) : count(0), threw(false) {
-    results.resize(n);
-  }
+  explicit CollectContext(int n) : count(0), threw(false) {}
+
   Promise<std::vector<T>> p;
   std::vector<T> results;
   std::atomic<size_t> count;