Harden Variant Reading

[steveloughran]

Feature Request / Improvement

Similar to apache/parquet-java#3561; Iceberg's variant code needs to be hardened against malformed files. They're already rejected ultimately, but range checking will fail before any memory allocations take place.

Query engine

None

Willingness to contribute

• I can contribute this improvement/feature independently
• I would be willing to contribute this improvement/feature with guidance from the Iceberg community
• I cannot contribute this improvement/feature at this time