Passing null to Repository.Discover(...) causes AccessViolationException / process termination

Reproduction steps

1. Pass null to Repository.Discover(...)

Expected behavior

A friendly exception. 😉

Actual behavior

Application: devenv.exe
Framework Version: v4.0.30319
Description: The process was terminated due to an unhandled exception.
Exception Info: System.AccessViolationException
   at LibGit2Sharp.Core.NativeMethods.git_repository_discover(LibGit2Sharp.Core.Handles.GitBuf, LibGit2Sharp.Core.FilePath, Boolean, LibGit2Sharp.Core.FilePath)
   at LibGit2Sharp.Core.Proxy+<>c__DisplayClass3e.<git_repository_discover>b__3d(LibGit2Sharp.Core.Handles.GitBuf)
   at LibGit2Sharp.Core.Proxy.ConvertPath(System.Func`2<LibGit2Sharp.Core.Handles.GitBuf,Int32>)
   at LibGit2Sharp.Core.Proxy.git_repository_discover(LibGit2Sharp.Core.FilePath)
   at LibGit2Sharp.Repository.Discover(System.String)
   at GitHub.Services.GitService.GetRepository(System.String)

Version of LibGit2Sharp (release number or SHA1)

0.23.1.0

Operating system(s) tested; .NET runtime tested

Windows 10 + .NET v4.0.30319

The failing entry point is Repository.Discover(String), which reaches Core.Proxy.git_repository_discover and NativeMethods.git_repository_discover in the stack trace. Start there by tracing how a null path is passed to the native call, then add a regression test and verify that null produces a managed exception instead of terminating the process.