Patch WinMM header structure

[Nytra]

Patched according to #88

I did not change the ints to uints because it would require more changes in other parts of the code.

This has not been tested.

[atsushieno]

Thanks for the fix!

[Psychlist1972]

I just tested this with .NET 10. Granted, that is likely a different version than you are using, but I wanted to check this because it didn't look correct.

1. Be sure to add Pack=1 otherwise you get the wrong structure size. I was getting a size of 120 on 64 bit compiles without this.
2. The [] after IntPtr are required to get a proper size for the struct, at least in .NET 10. In .NET 10, you get an exception if you don't include this.

You should end up with a size of 112. No more, no less.

[StructLayout(LayoutKind.Sequential, Pack =1)]
struct MidiHdr
{
    public IntPtr Data;
    public int BufferLength; // should be uint
    public int BytesRecorded; // should be uint
    public IntPtr User;
    public int Flags;
    public IntPtr Next; // of MidiHdr
    public IntPtr Reserved;
    public int Offset; // should be uint
    [MarshalAs(UnmanagedType.ByValArray, SizeConst = 8)]
    private IntPtr[] reservedArray;
}

[Psychlist1972]

Just to also be clear, the structure was always incorrect, and could have resulted in app crashes. What changed is we're now validating the passed-in size in the WinMM call. :)

[Nytra]

Thanks. This PR should fix it #90