builtin/stash - segmentation fault

Hi,

When `git  stash` on detached HEAD I  got a segmentation fault. I am
using git 2.45.2 on OSX/Darwin. Furthermore the culprit is
EXC_BAD_ACCESS on `repo_find_unique_abbrev_r` because `oid` is
invalid. The `oid` was obtained from NULL `lookup_commit` in
`do_create_stash`. The result from `lookup_commit` should be checked
for validation to avoid null access.

https://github.com/git/git/blob/7b0defb3915eaa0bd118f0996e8c00b4eb2dc1ca/builtin/stash.c#L1380

Thanks.