avformat/mp3dec: only call ffio_ensure_seekback once

Otherwise the subsequent ffio_ensure_seekback calls destroy the buffer of the
earlier. The worst case ~66kB seekback is so small it is easier to request it
entirely.

Fixes ticket #10837, a regression since
0d17f5228f.

Signed-off-by: Marton Balint <cus@passwd.hu>
(cherry picked from commit b005317219)

libavformat/mp3dec.c

@@ -32,6 +32,7 @@
 #include "replaygain.h"
 
 #include "libavcodec/codec_id.h"
+#include "libavcodec/mpegaudio.h"
 #include "libavcodec/mpegaudiodecheader.h"
 
 #define XING_FLAG_FRAMES 0x01
@@ -400,15 +401,16 @@ static int mp3_read_header(AVFormatContext *s)
     if (ret < 0)
         return ret;
 
+    ret = ffio_ensure_seekback(s->pb, 64 * 1024 + MPA_MAX_CODED_FRAME_SIZE + 4);
+    if (ret < 0)
+        return ret;
+
     off = avio_tell(s->pb);
     for (i = 0; i < 64 * 1024; i++) {
         uint32_t header, header2;
         int frame_size;
-        if (!(i&1023))
-            ffio_ensure_seekback(s->pb, i + 1024 + 4);
         frame_size = check(s->pb, off + i, &header);
         if (frame_size > 0) {
-            ffio_ensure_seekback(s->pb, i + 1024 + frame_size + 4);
             ret = check(s->pb, off + i + frame_size, &header2);
             if (ret >= 0 &&
                 (header & MP3_MASK) == (header2 & MP3_MASK))