pcap: fix Rx crash

pcap: fix Rx crash

Normally, bufs[i] stores the mbuf pointer, the index of buf[i]
is the loop count i, but if header.len > buf_size, DPDK will
free the mbuf, but the loop count i still increases, so some
of the items in bufs[] might be NULL pointer, causing a potential
DPDK core. Using num_rx as the index for bufs[] solves the problem.

Signed-off-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>
Tested-by: Jiajia SunX <sunx.jiajia@intel.com>
Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>