avcodec/jpeg2000: zero i/f_data

Fixes uninitialized use of variables

Fixes part of Ticket2921

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/jpeg2000dec: prevent out of array accesses in pixel addressing

Fixes Ticket2921

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