ptdump: fix raw memory dumping

With an earlier change to the behavior of pt_pkt_next(), ptdump's --raw option
had been off by the size of the packet to dump.  It was dumping the memory
behind the packet.  Fix it.

Change-Id: I91b498f587b0e1fa091ddb94ff6f87ce188a1050
Signed-off-by: Markus Metzger <markus.t.metzger@intel.com>