avformat/mp3dec: perform seek resync in the correct direction

Fixes seeking to the last frame in CBR files
Fixes Ticket2773
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/mp3dec.c

@@ -288,6 +288,7 @@ static int mp3_seek(AVFormatContext *s, int stream_index, int64_t timestamp,
     AVStream *st = s->streams[0];
     int64_t ret  = av_index_search_timestamp(st, timestamp, flags);
     int i, j;
+    int dir = (flags&AVSEEK_FLAG_BACKWARD) ? -1 : 1;
 
     if (mp3->is_cbr && st->duration > 0 && mp3->header_filesize > s->data_offset) {
         int64_t filesize = avio_size(s->pb);
@@ -317,7 +318,7 @@ static int mp3_seek(AVFormatContext *s, int stream_index, int64_t timestamp,
 
 #define MIN_VALID 3
     for(i=0; i<4096; i++) {
-        int64_t pos = ie->pos + i;
+        int64_t pos = ie->pos + i*dir;
         for(j=0; j<MIN_VALID; j++) {
             ret = check(s, pos);
             if(ret < 0)
@@ -330,7 +331,7 @@ static int mp3_seek(AVFormatContext *s, int stream_index, int64_t timestamp,
     if(j!=MIN_VALID)
         i=0;
 
-    ret = avio_seek(s->pb, ie->pos + i, SEEK_SET);
+    ret = avio_seek(s->pb, ie->pos + i*dir, SEEK_SET);
     if (ret < 0)
         return ret;
     ff_update_cur_dts(s, st, ie->timestamp);