Fix stack use after return in Observer.ObservableLockInversion

Reviewed By: praihan

Differential Revision: D32297368

fbshipit-source-id: a58c7805d396cc30c0d82a49d5998e2f9d8a799a

folly/experimental/observer/test/ObserverTest.cpp

@@ -951,7 +951,7 @@ TEST(Observer, ObservableLockInversion) {
 
   while (true) {
     std::lock_guard<std::mutex> lg(lockingObservableLock);
-    if (**makeObserver([&] { return **observer; }) == kNumIters) {
+    if (**makeObserver([o = observer] { return **o; }) == kNumIters) {
       break;
     }
   }