Fix out-of-bound reads in the LHS/RHS packing functions

- Make RHS packing function accessing in-bound elements when N is not
multiple of nr
- Make LHS packing function accessing in-bound elements when K is not
multiple of kr * sr

Signed-off-by: Gian Marco Iodice <gianmarco.iodice@arm.com>

Reviewed-by: Viet-Hoa Do <viet-hoa.do@arm.com>
Approved-by: Viet-Hoa Do <viet-hoa.do@arm.com>