fixed potential crash in dasher when using period continuity - cf #1726
fixed potential crash in dasher when using period continuity - cf #1726

rbouqueau/gpac,rbouqueau/gpac,rbouqueau/gpac,gpac/gpac,rbouqueau/gpac,gpac/gpac,gpac/gpac,gpac/gpac,gpac/gpac,gpac/gpac,gpac/gpac,rbouqueau/gpac,rbouqueau/gpac,rbouqueau/gpac,gpac/gpac,rbouqueau/gpac