Fix problem with multithreaded decoding, introduced by r11214.

Patch by Reinhard Nissl: rnissl gmx de

Originally committed as revision 11275 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/h264.c

@@ -3849,7 +3849,7 @@ static int decode_slice_header(H264Context *h, H264Context *h0){
     h->slice_type= slice_type;
 
     s->pict_type= h->slice_type; // to make a few old func happy, it's wrong though
-    if (s->pict_type == B_TYPE && s->last_picture_ptr == NULL) {
+    if (s->pict_type == B_TYPE && s0->last_picture_ptr == NULL) {
         av_log(h->s.avctx, AV_LOG_ERROR,
                "B picture before any references, skipping\n");
         return -1;