pdump: fix possible mbuf leak on failure

If pdump_pktmbuf_copy_data() fails it's possible to have segment leak
as rte_pktmbuf_free() only handles m_dup chain but not the seg just
allocated and yet not chained.

Fixes: 278f945402c5 ("pdump: add new library for packet capture")

Signed-off-by: Ilya V. Matveychikov <matvejchikov@gmail.com>