fix decoding of (broken) files with f_code=0

fix segfault if the first P frames header is damaged

Originally committed as revision 4432 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/mpeg12.c

@@ -2220,7 +2220,7 @@ static int mpeg1_decode_picture(AVCodecContext *avctx,
     if (s->pict_type == P_TYPE || s->pict_type == B_TYPE) {
         s->full_pel[0] = get_bits1(&s->gb);
         f_code = get_bits(&s->gb, 3);
-        if (f_code == 0)
+        if (f_code == 0 && avctx->error_resilience >= FF_ER_COMPLIANT)
             return -1;
         s->mpeg_f_code[0][0] = f_code;
         s->mpeg_f_code[0][1] = f_code;
@@ -2228,7 +2228,7 @@ static int mpeg1_decode_picture(AVCodecContext *avctx,
     if (s->pict_type == B_TYPE) {
         s->full_pel[1] = get_bits1(&s->gb);
         f_code = get_bits(&s->gb, 3);
-        if (f_code == 0)
+        if (f_code == 0 && avctx->error_resilience >= FF_ER_COMPLIANT)
             return -1;
         s->mpeg_f_code[1][0] = f_code;
         s->mpeg_f_code[1][1] = f_code;
@@ -3138,8 +3138,12 @@ static int mpeg_decode_frame(AVCodecContext *avctx,
                         start_code <= SLICE_MAX_START_CODE) {
                         int mb_y= start_code - SLICE_MIN_START_CODE;
                         
+                        if(s2->last_picture_ptr==NULL){
                         /* skip b frames if we dont have reference frames */
-                        if(s2->last_picture_ptr==NULL && s2->pict_type==B_TYPE) break;
+                            if(s2->pict_type==B_TYPE) break;
+                        /* skip P frames if we dont have reference frame no valid header */
+                            if(s2->pict_type==P_TYPE && !s2->first_slice) break;
+                        }
                         /* skip b frames if we are in a hurry */
                         if(avctx->hurry_up && s2->pict_type==B_TYPE) break;
                         /* skip everything if we are in a hurry>=5 */