[Java] Replace multiple calls to getShort in NettyArrowBuf

As per https://github.com/apache/arrow/pull/7347#discussion_r437116925 replace two calls to getShort with one

**Reporter**: [Ryan Murray](https://issues.apache.org/jira/browse/ARROW-9148) / @rymurr

_{**Note**: *This issue was originally created as [ARROW-9148](https://issues.apache.org/jira/browse/ARROW-9148). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

Start by locating the NettyArrowBuf class and the two getShort calls referenced in the issue. Confirm the surrounding logic and existing Java tests, then verify that the calls are replaced as requested and the relevant tests pass.

Written by the indexing model from the issue text.