Fix bug that uses &read for num bytes received

Summary:
Bug currently passes `size_t(read)`, which is the memory address of the `read` function. The correct argument is a positive `ret`, which is the length of the message in bytes.

The bug is innocuous because the value is placed in a log statement and otherwise unused.

I clean up the logic and only notify when `ret >= 0`.

Reviewed By: ot

Differential Revision: D66336105

fbshipit-source-id: ca2b95275048893aa5c1b86a5ffb534a806bc2ee

folly/io/async/test/AsyncUDPSocketTest.cpp

@@ -372,14 +372,18 @@ class UDPNotifyClient : public UDPClient {
       if (errno != EAGAIN || errno != EWOULDBLOCK) {
         onReadError(folly::AsyncSocketException(
             folly::AsyncSocketException::NETWORK_ERROR, "error"));
-        return;
       }
-    }
-    SocketAddress addr;
-    addr.setFromSockaddr(rawAddr, addrLen);
+    } else {
+      // Datagram sockets in various domains (e.g., the UNIX and Internet
+      // domains) permit zero-length datagrams.  When such a datagram is
+      // received, the return value is 0.
+      auto numBytesRecv{static_cast<size_t>(ret)};
 
-    onDataAvailable(
-        addr, size_t(folly::fileops::read), false, OnDataAvailableParams());
+      SocketAddress addr;
+      addr.setFromSockaddr(rawAddr, addrLen);
+
+      onDataAvailable(addr, numBytesRecv, false, OnDataAvailableParams());
+    }
   }
 
   void onRecvMmsg(AsyncUDPSocket& sock) {