## What changes were proposed in this pull request?

Use `BytesWritable.copyBytes`, not `getBytes`, because `getBytes` returns the underlying array, which may be reused when repeated reads don't need a different size, as is the case with binaryRecords APIs

## How was this patch tested?

Existing tests

Author: Sean Owen <sowen@cloudera.com>

Closes #16974 from srowen/SPARK-19646.

(cherry picked from commit d0ecca60)
Signed-off-by: Sean Owen <sowen@cloudera.com>