Make LibVLC disposal idempotent

src/LibVLCSharp.Tests/LibVLCTests.cs

@@ -119,6 +119,39 @@ public void DisposeLibVLC()
             Assert.That(_libVLC.DialogHandlersSet, Is.False);
         }
 
+        [Test]
+        public void DisposeLibVLCIsIdempotent()
+        {
+            const string childProcessVariable = "LIBVLCSHARP_DOUBLE_DISPOSE_CHILD";
+            if (Environment.GetEnvironmentVariable(childProcessVariable) == "1")
+            {
+                var childLibVLC = new LibVLC("--no-audio", "--no-video");
+                childLibVLC.Dispose();
+                childLibVLC.Dispose();
+                Assert.That(childLibVLC.NativeReference, Is.EqualTo(IntPtr.Zero));
+                return;
+            }
+
+            var startInfo = new ProcessStartInfo
+            {
+                FileName = Environment.GetEnvironmentVariable("DOTNET_HOST_PATH") ?? "dotnet",
+                UseShellExecute = false
+            };
+            startInfo.ArgumentList.Add(typeof(LibVLCTests).Assembly.Location);
+            startInfo.ArgumentList.Add($"--test={typeof(LibVLCTests).FullName}.{nameof(DisposeLibVLCIsIdempotent)}");
+            startInfo.ArgumentList.Add("--workers=1");
+            startInfo.ArgumentList.Add("--noheader");
+            startInfo.Environment[childProcessVariable] = "1";
+
+            using (var process = Process.Start(startInfo))
+            {
+                Assert.That(process, Is.Not.Null);
+                Assert.That(process.WaitForExit(60_000), Is.True, "The double-dispose child process should terminate");
+                Assert.That(process.ExitCode, Is.Zero,
+                    "Disposing LibVLC twice should not crash the process");
+            }
+        }
+
         [Test]
         public void LibVLCVersion()
         {

src/LibVLCSharp/Shared/LibVLC.cs

@@ -372,11 +372,15 @@ static string[] PatchOptions(string[] options, bool enableDebugLogs = false, boo
         /// <param name="disposing"></param>
         protected override void Dispose(bool disposing)
         {
+            if (IsDisposed)
+                return;
+
             if (disposing)
             {
                 UnsetDialogHandlers();
                 Native.LibVLCLogUnset(NativeReference);
-                _gcHandle.Free();
+                if (_gcHandle.IsAllocated)
+                    _gcHandle.Free();
                 _exitCallback = null;
                 _log = null;
 #if DESKTOP