Out-of-bounds write in nix::unistd::getgrouplist

Summary

On certain platforms, if a user has more than 16 groups, the `nix::unistd::getgrouplist` function will call the libc `getgrouplist` function with a length parameter greater than the size of the buffer it provides, resulting in an out-of-bounds write and memory corruption. The libc `getgrouplist` function takes an in/out parameter `ngroups` specifying the size of the group buffer. When the buffer is too small to hold all of the reqested user's group memberships, some libc implementations, including glibc and Solaris libc, will modify `ngroups` to indicate the actual number of groups for the user, in addition to returning an error. The version of `nix::unistd::getgrouplist` in nix 0.16.0 and up will resize the buffer to twice its size, but will not read or modify the `ngroups` variable. Thus, if the user has more than twice as many groups as the initial buffer size of 8, the next call to `getgrouplist` will then write past the end of the buffer. The issue would require editing /etc/groups to exploit, which is usually only editable by the root user.