fixed parsing issue and added test

Tests/MQTTnet.AspNetCore.Tests/ReaderExtensionsTest.cs

@@ -0,0 +1,45 @@
+using System.Buffers;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using MQTTnet.AspNetCore;
+using MQTTnet.Packets;
+using MQTTnet.Serializer;
+
+namespace MQTTnet.AspNetCore.Tests
+{
+    [TestClass]
+    public class ReaderExtensionsTest
+    {
+        [TestMethod]
+        public void TestTryDeserialize()
+        {
+            var serializer = new MqttPacketSerializer();
+
+            var buffer = serializer.Serialize(new MqttPublishPacket() {Topic = "a", Payload = new byte[5]});
+
+            var sequence = new ReadOnlySequence<byte>(buffer.Array, buffer.Offset, buffer.Count);
+
+            var part = sequence;
+            MqttBasePacket packet;
+            var consumed = part.Start;
+            var observed = part.Start;
+            var result = false;
+
+            part = sequence.Slice(sequence.Start, 0); // empty message should fail
+            result = serializer.TryDeserialize(part, out packet, out consumed, out observed);
+            Assert.IsFalse(result);
+
+
+            part = sequence.Slice(sequence.Start, 1); // partial fixed header should fail
+            result = serializer.TryDeserialize(part, out packet, out consumed, out observed);
+            Assert.IsFalse(result);
+
+            part = sequence.Slice(sequence.Start, 4); // partial body should fail
+            result = serializer.TryDeserialize(part, out packet, out consumed, out observed);
+            Assert.IsFalse(result);
+
+            part = sequence; // complete msg should work
+            result = serializer.TryDeserialize(part, out packet, out consumed, out observed);
+            Assert.IsTrue(result);
+        }
+    }
+}