[SPARK-12084][CORE] Fix codes that uses ByteBuffer.array incorrectly

`ByteBuffer` doesn't guarantee all contents in `ByteBuffer.array` are valid. E.g, a ByteBuffer returned by `ByteBuffer.slice`. We should not use the whole content of `ByteBuffer` unless we know that's correct.

This patch fixed all places that use `ByteBuffer.array` incorrectly.

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #10083 from zsxwing/bytebuffer-array.