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

從 Repository.IsValid 開始，追蹤其呼叫經過 Proxy.git_repository_open_ext 到 NativeMethods.git_repository_open_ext，然後檢查涉及 git_repository_free 的原生錯誤路徑。在 Windows 上重現 .git/config 損壞的情況，並驗證它引發的是 LibGit2SharpException，而不是存取違規；如果現有測試配置表明存在合適的位置，則為此情境新增或更新涵蓋範圍。