Fix out-of-buffer read on image upscale

Avoid reading from the next pixel when the sample is fully based on the
current pixel. This is particular important for the last pixel in an
image when the next pixel might be outside the image buffer.

Change-Id: I3607f9c6c332d11ff944ca35d216d417368f9fd5
Task-number: QTBUG-47228
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>
Reviewed-by: Gunnar Sletta <gunnar@sletta.org>

src/gui/painting/qimagescale.cpp

@@ -312,7 +312,10 @@ static void qt_qimageScaleAARGBA_up_xy(QImageScaleInfo *isi, unsigned int *dest,
             for (int x = dxx; x < end; x++) {
                 const unsigned int *pix = sptr + xpoints[x];
                 const int xap = xapoints[x];
+                if (xap > 0)
                     *dptr = INTERPOLATE_PIXEL_256(pix[0], 256 - xap, pix[1], xap);
+                else
+                    *dptr = pix[0];
                 dptr++;
             }
         }