avcodec/vp3: Replace check by assert

Fixes: CID1452425 Logically dead code

Sponsored-by: Sovereign Tech Fund
Reviewed-by: Peter Ross <pross@xvid.org>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 1b991e77b9)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/vp3.c

@@ -1999,8 +1999,7 @@ static int vp4_mc_loop_filter(Vp3DecodeContext *s, int plane, int motion_x, int
         x_offset = (-(x + 2) & 7) + 2;
         y_offset = (-(y + 2) & 7) + 2;
 
-        if (x_offset > 8 + x_subpel && y_offset > 8 + y_subpel)
-            return 0;
+        av_assert1(!(x_offset > 8 + x_subpel && y_offset > 8 + y_subpel));
 
         s->vdsp.emulated_edge_mc(loop, motion_source - stride - 1,
              loop_stride, stride,