Fix small ubsan issue in EventBaseAtomicNotificationQueue

Summary:
`sizeof()` produces an unsigned value, and `result` can be `-1`, so this comparison was technically UB. For extra fun, triggering a ubsan issue in this location changes `errno`. This probably shouldn't actually be detected as a ubsan issue, since the `sizeof()` is trivially known at compile time, but C++ semantics say otherwise :(

Fixes: #1953

Reviewed By: vitalii-topoliuk

Differential Revision: D44110957

fbshipit-source-id: f41f7d3ef9fb71c0e5368f5c4ebbabd814b91609

folly/io/async/EventBaseAtomicNotificationQueue-inl.h

@@ -221,7 +221,9 @@ void EventBaseAtomicNotificationQueue<Task, Consumer>::drainFd() {
   uint64_t message = 0;
   if (eventfd_ >= 0) {
     auto result = readNoInt(eventfd_, &message, sizeof(message));
-    CHECK(result == sizeof(message) || errno == EAGAIN || errno == EWOULDBLOCK)
+    CHECK(
+        result == (int)sizeof(message) || errno == EAGAIN ||
+        errno == EWOULDBLOCK)
         << "result = " << result << "; errno = " << errno;
     writesObserved_ += message;
   } else {