lavf/id3v2: seek back to previous offset if header size is not matched

Makes ff_id3v2_read reset stream position at the end of ID3 data if the
header size is not matched (caused by an EOF for example).

Current behaviour (without the patch):
    filesize = 400
    id3 data size = 399
    file offset after ff_id3v2_read is 400 instead of 399
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/id3v2.c

@@ -788,8 +788,10 @@ void ff_id3v2_read(AVFormatContext *s, const char *magic, ID3v2ExtraMeta **extra
         /* save the current offset in case there's nothing to read/skip */
         off = avio_tell(s->pb);
         ret = avio_read(s->pb, buf, ID3v2_HEADER_SIZE);
-        if (ret != ID3v2_HEADER_SIZE)
+        if (ret != ID3v2_HEADER_SIZE) {
+            avio_seek(s->pb, off, SEEK_SET);
             break;
+        }
         found_header = ff_id3v2_match(buf, magic);
         if (found_header) {
             /* parse ID3v2 header */