[Java] allocate new buffer code doesn't release extra allocated buffer properly

[Class BaseValueVector](https://github.com/apache/arrow/blob/master/java/vector/src/main/java/org/apache/arrow/vector/BaseValueVector.java) 's method allocFixedDataAndValidityBufs on line#162 allocates a buffer in power of 2 size. After that, it has a code to release extra buffer. For that, it calculates the extra buffer from allocated size "bufferSize" but in my opinion, it should take original "valueCount" to find the extra buffer size. 

Here, I see a problem in line#162, where its taking "bufferSize" to find the extra allocated bytes. It should be "valueCount\*typeWidth + valueCount/8".

Here is an example for that. Let's take 1000 ints. Then,
valueCount = 1000 ints
typeWidth = 4 bytes
validitiyBufferSize = 125 bytes
valueBufferSize = 4000 bytes
combinedSize(valueBufferSize + validityBufferSize) = 4128 bytes (multiple of 8)
combinedSizeWith2ThePowerSize = 8192 bytes, this will be "bufferSize" at line#152.

With the above calculation, this code should release (combinedSizeWith2ThePowerSize - combinedSize) = 4064 bytes. But, this is not happening.

**Reporter**: [Hitesh Khamesra](https://issues.apache.org/jira/browse/ARROW-5057)
#### PRs and other links:
- [GitHub Pull Request apache/arrow#4079](https://github.com/apache/arrow/pull/4079)

_{**Note**: *This issue was originally created as [ARROW-5057](https://issues.apache.org/jira/browse/ARROW-5057). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

Inizia in java/vector/src/main/java/org/apache/arrow/vector/BaseValueVector.java, in allocFixedDataAndValidityBufs, prestando particolare attenzione all’allocazione e al rilascio del buffer aggiuntivo intorno alla riga 162. Segui i calcoli delle dimensioni dei buffer per l’esempio con 1.000 interi e verifica che la modifica completata rilasci l’allocazione aggiuntiva prevista; nota che PR apache/arrow#4079 è già collegata a questa issue.

Scritto dal modello di indicizzazione a partire dal testo della issue.