net/pcap: fix crash on exit for infinite Rx

net/pcap: fix crash on exit for infinite Rx

If the infinite Rx argument ('infinite_rx') is provided a ring is
allocated and filled in the '.rx_queue_setup' dev_ops.
Later this ring freed in the '.dev_close' dev_ops.

If the 'infinite_rx' provided and '.dev_close' called before
'.rx_queue_setup', the ring will be NULL and trying to empty/free it
will cause a crash.

This is fixed by adding ring NULL check before trying to empty/free it.

Bugzilla ID: 548
Fixes: a3f5252e5cbd ("net/pcap: enable infinitely Rx a pcap file")
Cc: stable@dpdk.org
Signed-off-by: Ferruh Yigit <ferruh.yigit@intel.com>