Minor QByteArray::toBase64 improvement

While researching a bug, I came across a minor issue in QByteArray::toBase64. The size calculation overcalculates the size required for the output by up to 3 Bytes.

The code in the patch calculates the size properly, and removes the truncate() because it's now unnecessary.

And the usual disclaimer: the code I added is public domain, I disclaim any ownership or copyright.