fix: freed eventPtr in AdminClient (#1836)

Author: emasab

src/Confluent.Kafka/AdminClient.cs

@@ -131,10 +131,10 @@ private Task StartPollTask(CancellationToken ct)
                         while (true)
                         {
                             ct.ThrowIfCancellationRequested();
-
+                            IntPtr eventPtr = IntPtr.Zero;
                             try
                             {
-                                var eventPtr = kafkaHandle.QueuePoll(resultQueue, this.cancellationDelayMaxMs);
+                                eventPtr = kafkaHandle.QueuePoll(resultQueue, this.cancellationDelayMaxMs);
                                 if (eventPtr == IntPtr.Zero)
                                 {
                                     continue;
@@ -351,6 +351,13 @@ private Task StartPollTask(CancellationToken ct)
                                 this.DisposeResources();
                                 break;
                             }
+                            finally
+                            {
+                                if (eventPtr != IntPtr.Zero)
+                                {
+                                    Librdkafka.event_destroy(eventPtr);
+                                }
+                            }
                         }
                     }
                     catch (OperationCanceledException) {}