Crash calling Repository.IsValid with corrupt .git/config

Reproduction steps

Add this to .git/config
[diff "mytool"] command = c:\path\to\my\tool\MyToolComparer.exe
This will cause the following effect for git

c:\my\git\repo> git status
fatal: bad config line 8 in file .git/config

Then call Repository.IsValid(@"c:\my\git\repo\.git") which is turn calls Proxy.git_repository_open_ext(path, RepositoryOpenFlags.NoSearch, null);

This fails because the return value from NativeMethods.git_repository_open_ext(out repo, path, flags, ceilingDirs) is not checked for error prior to freeing the out parameter repo (which is already free'd when there is an error condition in int git_repository_open_ext(git_repository **repo_ptr, const char *start_path, unsigned int flags, const char *ceiling_dirs))

Expected behavior

LibGit2Sharp.LibGit2SharpException

Actual behavior

Access violation (0xC0000005) in void git_repository_free(git_repository *repo) when dereferencing the null repo

Version of LibGit2Sharp (release number or SHA1)

0.27.0.34

Operating system(s) tested; .NET runtime tested

Windows 10 Pro 1909 - .NET CLR Version v4.0.30319

从 Repository.IsValid 开始，跟踪其调用经过 Proxy.git_repository_open_ext 到 NativeMethods.git_repository_open_ext，然后检查涉及 git_repository_free 的原生错误路径。在 Windows 上复现 .git/config 损坏的情况，并验证它引发的是 LibGit2SharpException，而不是访问冲突；如果现有测试布局表明存在合适的位置，则为此场景添加或更新覆盖范围。