dds: limit 4 bpp handling to AV_PIX_FMT_PAL8

This fixes NULL pointer dereferencing for formats, where frame->data[1]
is not allocated.

The problem was introduced in commit
257fbc3af4cba08ac471dab68924182160bde6fd.

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
