When adding OIDs to the indexer's map of yet-to-be-seen OIDs to verify
that packfiles are complete, we do so by first allocating a new OID and
then calling `git_oidmap_set` on it. There was no check for memory
allocation errors in place, though, leading to possible segfaults due to
trying to copy data to a `NULL` pointer.
Verify the result of `git__malloc` with `GIT_ERROR_CHECK_ALLOC` to fix
the issue.
