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>

src/concurrent/qtconcurrentmedian.h

@@ -103,7 +103,7 @@ public:
             dirty = false;
             QVector<T> sorted = values;
             qSort(sorted);
-            currentMedian = sorted.at(bufferSize / 2 + 1);
+            currentMedian = sorted.at(bufferSize / 2);
         }
         return currentMedian;
     }