Fix bug in CQNext (#27022)

Internal bug b/170934515

In some cases, calling CQ::Next will return true without passing up a
new tag value, which is very much illegal. My expectation is that this
is due to messing up between SHUTDOWN and TIMEOUT in lower layer code
that doesn't particularly matter much to most callers, but was being
erroneously checked here.

include/grpcpp/impl/codegen/completion_queue.h

@@ -175,9 +175,14 @@ class CompletionQueue : private ::grpc::GrpcLibraryCodegen {
   /// \return true if got an event, false if the queue is fully drained and
   ///         shut down.
   bool Next(void** tag, bool* ok) {
+    // Check return type == GOT_EVENT... cases:
+    // SHUTDOWN  - queue has been shutdown, return false.
+    // TIMEOUT   - we passed infinity time => queue has been shutdown, return
+    //             false.
+    // GOT_EVENT - we actually got an event, return true.
     return (AsyncNextInternal(tag, ok,
                               ::grpc::g_core_codegen_interface->gpr_inf_future(
-                                  GPR_CLOCK_REALTIME)) != SHUTDOWN);
+                                  GPR_CLOCK_REALTIME)) == GOT_EVENT);
   }
 
   /// Read from the queue, blocking up to \a deadline (or the queue's shutdown).