## What changes were proposed in this pull request?

This patch fixes a bug in `KafkaSource` with the (de)serialization of the length of the JSON string that contains the initial partition offsets.

## How was this patch tested?

I ran the test suite for spark-sql-kafka-0-10.

Author: Roberto Agostino Vitillo <ra.vitillo@gmail.com>

Closes #16857 from vitillo/kafka_source_fix.