preserve parse context

Originally committed as revision 4241 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/mpeg12.c

@@ -2083,7 +2083,10 @@ static int mpeg_decode_postinit(AVCodecContext *avctx){
     {
     
         if (s1->mpeg_enc_ctx_allocated) {
+            ParseContext pc= s->parse_context;
+            s->parse_context.buffer=0;
             MPV_common_end(s);
+            s->parse_context= pc;
         }
 
 	if( (s->width == 0 )||(s->height == 0))