Reduce array allocations by usage MemoryStream.GetBuffer() instead of ToArray()

[lechu445]

No description provided.

[tonyqus]

Check this document

Note that the buffer contains allocated bytes which might be unused. For example, if the string "test" is written into the MemoryStream object, the length of the buffer returned from GetBuffer is 256, not 4, with 252 bytes unused. To obtain only the data in the buffer, use the ToArray method; however, ToArray creates a copy of the data in
memory.

The return bytes of GetBuffer may not be the same as that from ToArray.

[lahma]

I think in this PR the author is reading just the amount of Length so having "test" in the buffer and backing array being of size 256 is OK as only the elements 0-buffer.Length are being written?

[tonyqus]

@lechu445 Please check if you can fix the following unit test

_/testcases/main/POIFS/FileSystem/TestNPOIFSFileSystem.cs#L724
System.UnauthorizedAccessException : MemoryStream's internal buffer cannot be accessed.

[tonyqus]

LGTM