Fix use-after-free in WTCallback

Summary: `rBase` refers to `self->base_` on destruction, so we need to hold a reference to `self` for the whole function.

Reviewed By: agelun

Differential Revision: D49453402

fbshipit-source-id: 9259bcc15926244fa143c511ec73dfea29919ca2

folly/futures/WTCallback.h

@@ -84,7 +84,7 @@ struct WTCallback : public TBase::Callback {
     }
 
     (*rBase)->runInEventBaseThreadAlwaysEnqueue(
-        [self = std::move(self), ew = std::move(ew)]() mutable {
+        [self, ew = std::move(ew)]() mutable {
           self->cancelTimeout();
           // Don't need Promise anymore, break the circular reference
           auto promise = self->stealPromise();