## What changes were proposed in this pull request?
This PR fixes a random OOM issue occurred while running `ObjectHashAggregateSuite`.
This issue can be steadily reproduced under the following conditions:
1. The aggregation must be evaluated using `ObjectHashAggregateExec`;
2. There must be an input column whose data type involves `ArrayType` (an input column of `MapType` may even cause SIGSEGV);
3. Sort-based aggregation fallback must be triggered during evaluation.
The root cause is that while falling back to sort-based aggregation, we must sort and feed already evaluated partial aggregation buffers living in the hash map to the sort-based aggregator using an external sorter. However, the underlying mutable byte buffer of `UnsafeRow`s produced by the iterator of the external sorter is reused and may get overwritten when the iterator steps forward. After the last entry is consumed, the byte buffer points to a block of uninitialized memory filled by `5a`. Therefore, while reading an `UnsafeArrayData` out of the `UnsafeRow`, `5a5a5a5a` is treated as array size and triggers a memory allocation for a ridiculously large array and immediately blows up the JVM with an OOM.
To fix this issue, we only need to add `.copy()` accordingly.
## How was this patch tested?
New regression test case added in `ObjectHashAggregateSuite`.
Author: Cheng Lian <lian@databricks.com>
Closes#15976 from liancheng/investigate-oom.
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