QVarLengthArray is neither implicitly shared, nor does it feature a
magic resize() on out-of-bounds.
Therefore, data() doesn't detach(), so 't' remains stable.
The only reason for the copy, then, would be if T wasn't
self-assignment-safe, but we don't support such types.
Remove the copy.
Change-Id: I8dd12e1c9b8131ae17d641354fe362554062b78d
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
adcc68fd59