mss2: only use error correction for matching block counts

This fixes a heap-buffer-overflow in ff_er_frame_end when decoding mss2
with coded_width/coded_height larger than width/height.
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>

libavcodec/mss2.c

@@ -422,7 +422,13 @@ static int decode_wmv9(AVCodecContext *avctx, const uint8_t *buf, int buf_size,
 
     ff_vc1_decode_blocks(v);
 
+    if (v->end_mb_x == s->mb_width && s->end_mb_y == s->mb_height) {
         ff_er_frame_end(&s->er);
+    } else {
+        av_log(v->s.avctx, AV_LOG_WARNING,
+               "disabling error correction due to block count mismatch %dx%d != %dx%d\n",
+               v->end_mb_x, s->end_mb_y, s->mb_width, s->mb_height);
+    }
 
     ff_mpv_frame_end(s);