swscale/swscale: Fix several invalid shifts related to vChrDrop

Fixes: Invalid shifts
Fixes: #8166
Fixes: filter-crop_scale_vflip FATE-test

Reviewed-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libswscale/swscale.c

@@ -295,8 +295,8 @@ static int swscale(SwsContext *c, const uint8_t *src[],
         srcStride[2] =
         srcStride[3] = srcStride[0];
     }
-    srcStride[1] <<= c->vChrDrop;
-    srcStride[2] <<= c->vChrDrop;
+    srcStride[1] *= 1 << c->vChrDrop;
+    srcStride[2] *= 1 << c->vChrDrop;
 
     DEBUG_BUFFERS("swscale() %p[%d] %p[%d] %p[%d] %p[%d] -> %p[%d] %p[%d] %p[%d] %p[%d]\n",
                   src[0], srcStride[0], src[1], srcStride[1],