[main] Partial fix for AVAudioEngine.Connect(input, sink, format) crash when format == null (#9410)

* partial fix for https://github.com/xamarin/xamarin-macios/issues/9267

* simplify notequals

* add monotouch-test case

* add more using per manuel's feedback

Co-authored-by: Whitney Schmidt <whschm@microsoft.com>

src/AVFoundation/AVAudioFormat.cs

@@ -21,12 +21,16 @@ namespace AVFoundation {
 	public partial class AVAudioFormat {
 		public static bool operator == (AVAudioFormat a, AVAudioFormat b)
 		{
+			if ((object) a == (object) b)
+				return true;
+			if ((object) a == null ^ (object) b == null)
+				return false;
 			return a.Equals (b);
 		}
 		
 		public static bool operator != (AVAudioFormat a, AVAudioFormat b)
 		{
-			return !a.Equals (b);
+			return !(a == b);
 		}
 	}
 }

tests/monotouch-test/AVFoundation/AVAudioFormatTest.cs

@@ -0,0 +1,54 @@
+// Unit test for AVAudioFormat
+// Authors: 
+// 		Whitney Schmidt (whschm@microsoft.com)
+// Copyright 2020 Microsoft Corp.
+
+using System;
+using Foundation;
+using AVFoundation;
+using NUnit.Framework;
+namespace MonoTouchFixtures.AVFoundation {
+
+	[TestFixture]
+	[Preserve (AllMembers = true)]
+	public class AVAudioFormatTest {
+
+		[Test]
+		public void TestEqualOperatorSameInstace ()
+		{
+			using (var format = new AVAudioFormat ())
+				Assert.IsTrue (format == format, "format == format");
+		}
+
+		[Test]
+		public void TestEqualOperatorNull ()
+		{
+			using (var format = new AVAudioFormat ())
+			{
+				Assert.IsFalse (format == null, "format == null");
+				Assert.IsFalse (null == format, "null == format");
+			}
+			using (AVAudioFormat nullFormat = null)
+			{
+				Assert.IsTrue (nullFormat == null, "nullFormat == null");
+				Assert.IsTrue (null == nullFormat, "null == nullFormat");
+			}
+		}
+
+		[Test]
+		public void TestNotEqualOperatorNull ()
+		{
+			using (var format = new AVAudioFormat ())
+			{
+				Assert.IsTrue (format != null, "format != null");
+				Assert.IsTrue (null != format, "null != format");
+			}
+			using (AVAudioFormat nullFormat = null)
+			{
+				Assert.IsFalse (nullFormat != null, "nullFormat != null");
+				Assert.IsFalse (null != nullFormat, "null != nullFormat");
+			}
+
+		}
+	}
+}