Free the memory of the patched v-tables

Fixes https://github.com/mono/CppSharp/issues/586.

Signed-off-by: Dimitar Dobrev <dpldobrev@protonmail.com>

src/Generator/Generators/CSharp/CSharpSources.cs

@@ -1528,6 +1528,11 @@ namespace CppSharp.Generators.CSharp
             NewLine();
 
             GenerateVTableClassSetup(@class, wrappedEntries);
+            WriteLine("private static readonly global::System.Collections.Generic.List<" +
+                "global::CppSharp.Runtime.SafeUnmanagedMemoryHandle> __handleManagedVTables" +
+                " = new global::System.Collections.Generic.List<" +
+                "global::CppSharp.Runtime.SafeUnmanagedMemoryHandle>();");
+            NewLine();
 
             WriteLine("#endregion");
             PopBlock(NewLineKind.BeforeNextBlock);
@@ -1638,6 +1643,7 @@ namespace CppSharp.Generators.CSharp
                 string vfptr = $"vfptr{(destructorOnly ? "_dtor" : string.Empty)}{i}";
                 WriteLine($@"var {vfptr} = Marshal.AllocHGlobal({size} * {
                     Context.TargetInfo.PointerWidth / 8});");
+                WriteLine($"__handleManagedVTables.Add(new global::CppSharp.Runtime.SafeUnmanagedMemoryHandle({vfptr}, true));");
                 WriteLine($"{managedVTables}[{i}] = {vfptr}.ToPointer();");
 
                 AllocateNewVTableEntries(vftable.Layout.Components, wrappedEntries,
@@ -1655,6 +1661,7 @@ namespace CppSharp.Generators.CSharp
             int size = @class.Layout.Layout.Components.Count;
             uint pointerSize = Context.TargetInfo.PointerWidth / 8;
             WriteLine($"var vtptr{suffix} = Marshal.AllocHGlobal({size} * {pointerSize});");
+            WriteLine($"__handleManagedVTables.Add(new global::CppSharp.Runtime.SafeUnmanagedMemoryHandle(vtptr{suffix}, true));");
 
             WriteLine($@"var vfptr{suffix}0 = vtptr{suffix} + {
                 VTables.ItaniumOffsetToTopAndRTTI} * {pointerSize};");

src/Runtime/SafeUnmanagedMemoryHandle.cs

@@ -0,0 +1,28 @@
+using System;
+using System.Runtime.InteropServices;
+using System.Security.Permissions;
+using Microsoft.Win32.SafeHandles;
+
+namespace CppSharp.Runtime
+{
+    // https://stackoverflow.com/a/17563315/
+    [SecurityPermission(SecurityAction.Demand, UnmanagedCode = true)]
+    public sealed class SafeUnmanagedMemoryHandle : SafeHandleZeroOrMinusOneIsInvalid
+    {
+        public SafeUnmanagedMemoryHandle() : base(true) { }
+
+        public SafeUnmanagedMemoryHandle(IntPtr preexistingHandle, bool ownsHandle)
+            : base(ownsHandle) => SetHandle(preexistingHandle);
+
+        protected override bool ReleaseHandle()
+        {
+            if (handle != IntPtr.Zero)
+            {
+                Marshal.FreeHGlobal(handle);
+                handle = IntPtr.Zero;
+                return true;
+            }
+            return false;
+        }
+    }
+}