From: Kevin Backhouse <kevinbackhouse@github.com>

To reproduce (with an ASAN build):

```
mkdir test
cd test
git init
echo x > x.txt
git add .
git commit -m "WIP"
git checkout -b dev
echo y > x.txt
git add .
git commit -m "WIP"
git checkout main
echo z > x.txt
git add .
git commit -m "WIP"
echo a > x.txt
git add .
git merge dev
```

The fix is to call free_commit_list(merge_bases) when an error occurs.

Signed-off-by: Kevin Backhouse <kevinbackhouse@github.com>

Submitted-As: https://lore.kernel.org/git/pull.1577.git.1692389061490.gitgitgadget@gmail.com