QtConcurrent: fix median calculation

The median, for arrays of odd size, is found in [size/2], not [size/2+1].
For arrays of even size, the code doesn't work anyway, so nothing to do there.

Change-Id: Id23ff3fe9538c2d8ef8f88e23127cb92af08b444
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>