This fixes a NULL pointer dereference if vst->duration is 0.

The problem was introduced in commit 0588acaf.
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 86d00ede)
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>