Fix size miscalculation in QByteArray::toBase64

The size calculation in QByteArray::toBase64 overcalculates the size
required for the output by up to 3 Bytes. This is fixed, which also
implies that truncate() at the end is needed only if OmitTrailingEquals
is used.

Task-number: QTBUG-32436
Change-Id: I92a893047e7aca027c4aa0a6655bcca514585ff5
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>