net/e1000: fix crash on Tx done clean up
net/e1000: fix crash on Tx done clean up

As tx mbuf is not set for some advanced descriptors, if there is no
mbuf checking before rte_pktmbuf_free_seg() function be called on
the process of tx done clean up, that will cause a segfault. So add
a NULL pointer check to fix it.

Bugzilla ID: 501
Fixes: 8d907d2b79f7 ("net/igb: free consumed Tx buffers on demand")
Cc: stable@dpdk.org

Signed-off-by: Jeff Guo <62aa901f8680899d1e8b3d9b45cf4843c2976e1c@intel.com>
Reviewed-by: Wei Zhao <55941fd0e75edbdf274af2cfac8f147525678f08@intel.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk