Bug Report: panic in VDiff leads to `vttablet` crash

### Overview of the Issue

When mismatched rows are encountered by VDiff, an out of bounds access can cause vttablet to panic and crash.

### Reproduction Steps

n/a

### Binary Version

```sh
Verified to happen on v21, but likely to happen on v20 and v22 as well.
```

### Operating System and Environment details

```sh
n/a
```

### Log Fragments

```sh

```