fix AtomicHashMap race condition

Summary: AHM::find checks that the returned SimpleRetT index is >= numMapsAllocated_ but that value may have changed and find pick the first element in the next sub map. Use success instead.

@override-unit-failures

Test Plan: unit tests

Reviewed By: delong.j@fb.com

FB internal diff: D1126684

folly/AtomicHashMap-inl.h

@@ -155,7 +155,7 @@ typename AtomicHashMap<KeyT, ValueT, HashFcn, EqualFcn, Allocator>::iterator
 AtomicHashMap<KeyT, ValueT, HashFcn, EqualFcn, Allocator>::
 find(KeyT k) {
   SimpleRetT ret = findInternal(k);
-  if (ret.i >= numMapsAllocated_.load(std::memory_order_acquire)) {
+  if (!ret.success) {
     return end();
   }
   SubMap* subMap = subMaps_[ret.i].load(std::memory_order_relaxed);