Demote packet doesn't decrease wire length

[rsanger]

There is likely a bug in demote_packet(), when removing a Linux SLL header (which libtrace treats as a link-layer header) from a packet the code doesn't decrease the wire length only the capture length. I've only added code to decrease it in the pcap: case to reverse the actions applied by promote_packet().

See:

libtrace/lib/linktypes.c

Line 472 in cd8cd74

/* TODO: should we change the wirelen here too? */

cd8cd74