Fix undefined behavior in line drawing.

Left shift of negative values is undefined.

modules/imgproc/src/drawing.cpp

@@ -655,7 +655,7 @@ Line2( Mat& img, Point2l pt1, Point2l pt2, const void* color)
         pt1.y ^= pt2.y & j;
 
         x_step = XY_ONE;
-        y_step = (dy << XY_SHIFT) / (ax | 1);
+        y_step = dy * (1 << XY_SHIFT) / (ax | 1);
         ecount = (int)((pt2.x - pt1.x) >> XY_SHIFT);
     }
     else
@@ -668,7 +668,7 @@ Line2( Mat& img, Point2l pt1, Point2l pt2, const void* color)
         pt2.y ^= pt1.y & i;
         pt1.y ^= pt2.y & i;
 
-        x_step = (dx << XY_SHIFT) / (ay | 1);
+        x_step = dx * (1 << XY_SHIFT) / (ay | 1);
         y_step = XY_ONE;
         ecount = (int)((pt2.y - pt1.y) >> XY_SHIFT);
     }