commit/payload-link: Ensure we don't overrun target_checksum size

commit/payload-link: Ensure we don't overrun target_checksum size

This is another warning from recently changed code from Coverity:

```
1. Defect type: OVERRUN
16. libostree-2024.7/src/libostree/ostree-repo-commit.c:823:7: overrun-buffer-arg: Overrunning array "target_checksum" of 65 bytes by passing it to a function which accesses it at byte offset 258 using argument "size" (which evaluates to 259). [Note: The source code implementation of the function has been overridden by a builtin model.]
```

I think this can only happen if the repository is corrupt; the
data shouldn't be that long. But fix this by passing the max
length we expect; this will ignore the rest currently.

Signed-off-by: Colin Walters <walters@verbum.org>