demux: ogg: Don't read skeleton if no bones first

(cherry picked from commit 83a873ba0ea9fcf42a4bb02ffc68380321321d2e)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/demux/ogg.c

@@ -2995,7 +2995,7 @@ static void Ogg_ReadSkeletonIndex( demux_t *p_demux, ogg_packet *p_oggpacket )
             break;
         }
     }
-    if ( !p_stream ) return;
+    if ( !p_stream || !p_stream->p_skel ) return;
     uint64_t i_keypoints = GetQWLE( &p_oggpacket->packet[10] );
     msg_Dbg( p_demux, "%" PRIi64 " index data for %" PRIi32, i_keypoints, i_serialno );
     if ( !i_keypoints ) return;