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GH-34831: [C++] Check REE child buffers are valid before other checks (#34833)

### Rationale for this change

See issue. Invalid buffers can cause segfaults when `ValidateRunEndEncodedChildren` is called, so we need to validate the buffers first.

### What changes are included in this PR?

Move up buffer checks.

### Are these changes tested?

Checked against the fuzzer failure case that triggered the issue.

### Are there any user-facing changes?

**This PR contains a "Critical Fix".**
* Closes: #34831

Authored-by: Will Jones <willjones127@gmail.com>
Signed-off-by: Will Jones <willjones127@gmail.com>