vp3: correctly clip vp3_draw_horiz_band call

Originally committed as revision 22377 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/vp3.c

@@ -1565,7 +1565,7 @@ static void render_slice(Vp3DecodeContext *s, int slice)
       *     dispatch (slice - 1);
       */
 
-    vp3_draw_horiz_band(s, 64*slice + 64-16);
+    vp3_draw_horiz_band(s, FFMIN(64*slice + 64-16, s->height-16));
 }
 
 /*