"Read out of buffer" in DecodeForwardIteratorRecord

One of the PDB's I'm processing throws this InvalidOperationException.

```
Microsoft.DiaSymReader.Converter.dll!Microsoft.CodeAnalysis.Debugging.CustomDebugInfoReader.ReadInt16(System.Collections.Immutable.ImmutableArray bytes, ref int offset) Line 551 C#
Microsoft.DiaSymReader.Converter.dll!Microsoft.CodeAnalysis.Debugging.CustomDebugInfoReader.DecodeForwardIteratorRecord(System.Collections.Immutable.ImmutableArray bytes) Line 209 C#
Microsoft.DiaSymReader.Converter.dll!Microsoft.DiaSymReader.Tools.PdbConverterWindowsToPortable.Convert(Microsoft.DiaSymReader.ISymUnmanagedReader5 symReader, System.Reflection.PortableExecutable.PEReader peReader, System.IO.Stream targetPdbStream, System.Reflection.Metadata.BlobContentId pdbId) Line 311 C#
Microsoft.DiaSymReader.Converter.dll!Microsoft.DiaSymReader.Tools.PdbConverterWindowsToPortable.Convert(System.Reflection.PortableExecutable.PEReader peReader, System.IO.Stream sourcePdbStream, System.IO.Stream targetPdbStream) Line 55 C#
Microsoft.DiaSymReader.Converter.dll!Microsoft.DiaSymReader.Tools.PdbConverter.ConvertWindowsToPortable(System.Reflection.PortableExecutable.PEReader peReader, System.IO.Stream sourcePdbStream, System.IO.Stream targetPdbStream) Line 91 C#
```

This is the line:

https://github.com/dotnet/symreader-converter/blob/106d9556e641d96f944c0b999ae39b69191893d1/src/Microsoft.DiaSymReader.Converter/PdbConverterWindowsToPortable.cs#L311

I'm not seeing a file for `Microsoft.CodeAnalysis.Debugging.CustomDebugInfoReader` in this repo somehow even though it's in `Microsoft.DiaSymReader.Converter.dll`.

It's a Windows PDB. I can share it privately. I wouldn't expect it to be invalid because was compiled recently by a major software tools vendor.