[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.*}

Beginne in java/vector/src/main/java/org/apache/arrow/vector/BaseValueVector.java bei allocFixedDataAndValidityBufs, insbesondere bei der Allokation und der Freigabe des zusätzlichen Puffers um Zeile 162. Verfolge die Berechnungen der Puffergröße für das Beispiel mit 1.000 Ints und überprüfe, dass die abgeschlossene Änderung die erwartete zusätzliche Allokation freigibt; beachte, dass PR apache/arrow#4079 bereits mit diesem Issue verknüpft ist.

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