mpeg12dec: Prevent f_code from becoming invalid.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/mpeg12.c

@@ -1368,6 +1368,7 @@ static int mpeg1_decode_picture(AVCodecContext *avctx,
         f_code = get_bits(&s->gb, 3);
         if (f_code == 0 && (avctx->err_recognition & (AV_EF_BITSTREAM|AV_EF_COMPLIANT)))
             return -1;
+        f_code += !f_code;
         s->mpeg_f_code[0][0] = f_code;
         s->mpeg_f_code[0][1] = f_code;
     }
@@ -1376,6 +1377,7 @@ static int mpeg1_decode_picture(AVCodecContext *avctx,
         f_code = get_bits(&s->gb, 3);
         if (f_code == 0 && (avctx->err_recognition & (AV_EF_BITSTREAM|AV_EF_COMPLIANT)))
             return -1;
+        f_code += !f_code;
         s->mpeg_f_code[1][0] = f_code;
         s->mpeg_f_code[1][1] = f_code;
     }
@@ -1536,6 +1538,11 @@ static void mpeg_decode_picture_coding_extension(Mpeg1Context *s1)
         s->current_picture.f.pict_type = s->pict_type;
         s->current_picture.f.key_frame = s->pict_type == AV_PICTURE_TYPE_I;
     }
+    s->mpeg_f_code[0][0] += !s->mpeg_f_code[0][0];
+    s->mpeg_f_code[0][1] += !s->mpeg_f_code[0][1];
+    s->mpeg_f_code[1][0] += !s->mpeg_f_code[1][0];
+    s->mpeg_f_code[1][1] += !s->mpeg_f_code[1][1];
+
     s->intra_dc_precision         = get_bits(&s->gb, 2);
     s->picture_structure          = get_bits(&s->gb, 2);
     s->top_field_first            = get_bits1(&s->gb);