demux: mp4: point to atom's position of base offset is moov (fix #11746)

Not so common flags, lack of samples.
Unsure if it doesn't break something else.

(cherry picked from commit 4fe0751b76f43b9b0ac6aab34319a94e498536cd)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/demux/mp4/mp4.c

@@ -4540,7 +4540,7 @@ static bool AddFragment( demux_t *p_demux, MP4_Box_t *p_moox )
         }
         else if ( BOXDATA(p_tfhd)->i_flags & MP4_TFHD_DEFAULT_BASE_IS_MOOF )
         {
-            i_traf_base_data_offset = p_new->p_moox->i_pos + 8;
+            i_traf_base_data_offset = p_new->p_moox->i_pos /* + 8*/;
         }
         else
         {