avformat/img2dec: do not rewind custom io buffers

Fixes double free with some applications

Fixes vlc ticket14121
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/img2dec.c

@@ -341,6 +341,9 @@ int ff_img_read_header(AVFormatContext *s1)
                     break;
                 }
             }
+            if (s1->flags & AVFMT_FLAG_CUSTOM_IO) {
+                avio_seek(s1->pb, 0, SEEK_SET);
+            } else
                 ffio_rewind_with_probe_data(s1->pb, &probe_buffer, probe_buffer_size);
         }
         if (st->codec->codec_id == AV_CODEC_ID_NONE)