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GH-43414: [C++][Compute] Fix invalid memory access when resizing var-length buffer in row table (#43415)



### Rationale for this change

As #43414 explained. The UT in PR reproduces this issue well (may need ASAN to detect).

### What changes are included in this PR?

Check if `is_fixed_length` before treating the second buffer as offset.

### Are these changes tested?

UT included.

### Are there any user-facing changes?

None.

* GitHub Issue: #43414

Lead-authored-by: Ruoxi Sun <zanmato1984@gmail.com>
Co-authored-by: Felipe Oliveira Carvalho <felipekde@gmail.com>
Signed-off-by: Felipe Oliveira Carvalho <felipekde@gmail.com>