avcodec/mpeg4videodec: Avoid possibly aliasing violating casts

Found-by: kierank
Reviewed-by: Kieran Kunhya <kieran618@googlemail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/mpeg4videodec.c

@@ -1256,10 +1256,12 @@ not_coded:
  */
 static int mpeg4_decode_partitioned_mb(MpegEncContext *s, int16_t block[6][64])
 {
-    Mpeg4DecContext *ctx = (Mpeg4DecContext *)s;
+    Mpeg4DecContext *ctx = s->avctx->priv_data;
     int cbp, mb_type;
     const int xy = s->mb_x + s->mb_y * s->mb_stride;
 
+    av_assert2(s == (void*)ctx);
+
     mb_type = s->current_picture.mb_type[xy];
     cbp     = s->cbp_table[xy];
 
@@ -1341,12 +1343,13 @@ static int mpeg4_decode_partitioned_mb(MpegEncContext *s, int16_t block[6][64])
 
 static int mpeg4_decode_mb(MpegEncContext *s, int16_t block[6][64])
 {
-    Mpeg4DecContext *ctx = (Mpeg4DecContext *)s;
+    Mpeg4DecContext *ctx = s->avctx->priv_data;
     int cbpc, cbpy, i, cbp, pred_x, pred_y, mx, my, dquant;
     int16_t *mot_val;
     static const int8_t quant_tab[4] = { -1, -2, 1, 2 };
     const int xy = s->mb_x + s->mb_y * s->mb_stride;
 
+    av_assert2(s ==  (void*)ctx);
     av_assert2(s->h263_pred);
 
     if (s->pict_type == AV_PICTURE_TYPE_P ||