music_ogg: fix sdl_seek_func() return value (SDL-2.0 / main)

[sezero]

SDL_RWseek(), in both SDL-1.2 and SDL-2.0, returns the current offset upon success and not NOERROR (0), therefore it behaves like lseek, not fseek which vorbis expects. The current version seems to have worked so far by luck.

CC: @Wohlstand

[Wohlstand]

Interesting... Usually the seek doesn't fail until the out-of-range value has been requested, or disk failed for any reason (like, got been disconnected or unmounted for any unexpected error, etc.). Normally I never met any seek errors, but could try...

EDIT: Yes, I confirm this (what going at Vorbis itself):

static int _seek_helper(OggVorbis_File *vf,ogg_int64_t offset){
  if(vf->datasource){
    /* only seek if the file position isn't already there */
    if(vf->offset != offset){
      if(!(vf->callbacks.seek_func)||
         (vf->callbacks.seek_func)(vf->datasource, offset, SEEK_SET) == -1) // <--------------------
        return OV_EREAD;
      vf->offset=offset;
      ogg_sync_reset(&vf->oy);
    }
  }else{
    /* shouldn't happen unless someone writes a broken callback */
    return OV_EFAULT;
  }
  return 0;
}