av_samples_alloc_array_and_samples: Fix sizeof type.

Fixes CID1005317
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
mxf_timestamp_to_str: dont leave uninitilaized fields in time.

Fixes CID1005318
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>