GHSA-j39j-6gw9-jw6h: git2 has potential undefined behavior when dereferencing Buf struct

If the Buf struct is dereferenced immediately after calling new() or default() on the Buf struct, a null pointer is passed to the unsafe function slice::from_raw_parts. According to the safety section documentation of the function, data must be non-null and aligned even for zero-length slices or slices of ZSTs. Thus, passing a null pointer will lead to undefined behavior.