net: shell: Validate pointer provided with net pkt command

The net_pkt pointer provided to net pkt commands was not validated in
any way. Therefore it was fairly easy to crash an application by
providing invalid address.

This commit adds the pointer validation. It's checked whether the
pointer provided belongs to any net_pkt pools known to the net stack,
and if the pointer offset within the slab actually points to the
beginning of the net_pkt structure.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>