Hang in System.Buffer Memmove

When trying to convert a particular Windows PDB to portable, the method hangs. Stack trace and repro files attached.
[AWSSDK.2.3.55.2.nupkg.zip](https://github.com/dotnet/symreader-converter/files/4603903/AWSSDK.2.3.55.2.nupkg.zip) [rename to remove the .zip, it's a nupkg file].

It's using `lib\net35\AWSSDK.dll` as the PE stream and `lib\net35\AWSSDK.pdb` as the PDB.

It's using
.NET Core SDK (reflecting any global.json):
Version: 5.0.100-preview.3.20216.6
Commit: 9f62a32109

```
[Managed to Native Transition]
System.Private.CoreLib.dll!System.Buffer._Memmove(ref byte dest, ref byte src, ulong len) Unknown
System.Private.CoreLib.dll!System.Buffer.Memmove(ref byte dest, ref byte src, ulong len) Unknown
System.Private.CoreLib.dll!System.Runtime.InteropServices.Marshal.CopyToNative(byte[] source, int startIndex, System.IntPtr destination, int length) Unknown
System.Reflection.Metadata.dll!System.Reflection.Internal.StreamExtensions.CopyTo(System.IO.Stream source, byte* destination, int size) Unknown
System.Reflection.Metadata.dll!System.Reflection.Internal.StreamMemoryBlockProvider.ReadMemoryBlockNoLock(System.IO.Stream stream, bool isFileStream, long start, int size) Unknown
System.Reflection.Metadata.dll!System.Reflection.Internal.StreamMemoryBlockProvider.GetMemoryBlockImpl(int start, int size) Unknown
System.Reflection.Metadata.dll!System.Reflection.PortableExecutable.PEReader.GetPESectionBlock(int index) Unknown
System.Reflection.Metadata.dll!System.Reflection.PortableExecutable.PEReader.GetSectionData(int relativeVirtualAddress) Unknown
System.Reflection.Metadata.dll!System.Reflection.Metadata.PEReaderExtensions.GetMethodBody(System.Reflection.PortableExecutable.PEReader peReader, int relativeVirtualAddress) Unknown
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) Unknown
Microsoft.DiaSymReader.Converter.dll!Microsoft.DiaSymReader.Tools.PdbConverterWindowsToPortable.Convert(System.Reflection.PortableExecutable.PEReader peReader, System.IO.Stream sourcePdbStream, System.IO.Stream targetPdbStream) Unknown
Microsoft.DiaSymReader.Converter.dll!Microsoft.DiaSymReader.Tools.PdbConverter.ConvertWindowsToPortable(System.Reflection.PortableExecutable.PEReader peReader, System.IO.Stream sourcePdbStream, System.IO.Stream targetPdbStream) Unknown
Microsoft.DiaSymReader.Converter.dll!Microsoft.DiaSymReader.Tools.PdbConverter.ConvertWindowsToPortable(System.IO.Stream peStream, System.IO.Stream sourcePdbStream, System.IO.Stream targetPdbStream) Unknown
> Core.dll!NuGetPe.AssemblyMetadata.AssemblyDebugParser.AssemblyDebugParser(System.IO.Stream peStream, System.IO.Stream pdbStream) Line 36 C#

```