Fix index_entries pos:

It was being set wrong for files with data_offset > 0

Patch by Michael Chinen, mchinen gmail

Originally committed as revision 25239 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/avcodec.h

@@ -3738,6 +3738,8 @@ typedef struct AVCodecParserContext {
     int flags;
 #define PARSER_FLAG_COMPLETE_FRAMES           0x0001
 #define PARSER_FLAG_ONCE                      0x0002
+/// Set if the parser has a valid file offset
+#define PARSER_FLAG_FETCHED_OFFSET            0x0004
 
     int64_t offset;      ///< byte offset from starting packet start
     int64_t cur_frame_end[AV_PARSER_PTS_NB];