avcodec/takdec: Always initialize hsize, silence "may be used uninitialized" warning

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/mjpegdec: zero gb to silence warning about it being possibly uninitialized

The code is not speed relevant, also its more robust if the pointers are NULL instead of random.

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