examples/ip_pipeline: fix null packet in source port

examples/ip_pipeline: fix null packet in source port

This patch fixes the NULL packet processing problem. Originally,
pipeline's write attempt to NULL packets generated by source
port may crash the application.

This patch fixes the problem by enforcing each source port
defined in cfg file containing a user specified or default pcap
file path.

Fixes: 0e1e7d53 ("examples/ip_pipeline: add pcap file source")

Signed-off-by: Fan Zhang <roy.fan.zhang@intel.com>
Acked-by: Cristian Dumitrescu <cristian.dumitrescu@intel.com>