tidb_query_vec_executors: Fixed stack-borrowing undefined-behavior

I don't understand the stacked borrowing rules enough to understand why the
existing code is a violation. It is creating a mutable unsafe pointer; then
taking some immutable borrows to the same data while doing nothing with that
pointer; then later using the pointer.

Simply moving the creation of the unsafe pointer to immediately before it
is used removes the error.

Found with miri, which reports:

```
error: Undefined Behavior: trying to reborrow for SharedReadOnly, but parent tag <1121039> does not have an appropriate item in the borrow sta
ck
   --> components/tidb_query_vec_executors/src/slow_hash_aggr_executor.rs:281:32
    |
281 |             let offset_begin = self.group_key_buffer.len();
    |                                ^^^^^^^^^^^^^^^^^^^^^ trying to reborrow for SharedReadOnly, but parent tag <1121039> does not have an ap
propriate item in the borrow stack
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the rules it violated are still experimental
    = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information
```

Signed-off-by: Brian Anderson <[email protected]>

components/tidb_query_vec_executors/src/slow_hash_aggr_executor.rs

@@ -277,7 +277,6 @@ impl<Src: BatchExecutor> AggregationExecutorImpl<Src> for SlowHashAggregationImp
  )?;
  }
 
- let buffer_ptr = (&*self.group_key_buffer).into();
  for logical_row_idx in 0..logical_rows_len {
  let offset_begin = self.group_key_buffer.len();
 
@@ -361,6 +360,7 @@ impl<Src: BatchExecutor> AggregationExecutorImpl<Src> for SlowHashAggregationImp
  }
  }
 
+ let buffer_ptr = (&*self.group_key_buffer).into();
  // Extra column is not included in `GroupKeyRefUnsafe` to avoid being aggr on.
  let group_key_ref_unsafe = GroupKeyRefUnsafe {
  buffer_ptr,