matroskadec: set default duration for simple block

When a normal Block is parsed, duration is initialized to
AV_NOPTS_VALUE.  If it is not changed, then the track's default
duration is used.  But for SimpleBlock, duration is initialized to
0 instead of AV_NOPTS_VALUE.  This is due to the difference in how
EBML_NEST vs EBML_PASS are processed.  Setting duration to 0 leads
eventually to wrongly estimate the frame duration in util.c

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
