net/ixgbe: fix crash on detach

net/ixgbe: fix crash on detach

When detaching a port bound to ixgbe PMD, if the port
does not have any VFs, *vfinfo is not set and there is
a NULL dereference attempt, when calling
rte_eth_switch_domain_free(), which expects VFs to be used,
causing a segmentation fault.

Steps to reproduce:

./testpmd -- -i
testpmd> port stop all
testpmd> port close all
testpmd> port detach 0

Bugzilla ID: 57
Fixes: cf80ba6e2038 ("net/ixgbe: add support for representor ports")
Cc: stable@dpdk.org
Reported-by: Anatoly Burakov <anatoly.burakov@intel.com>
Signed-off-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>
Tested-by: Anatoly Burakov <anatoly.burakov@intel.com>
Acked-by: Remy Horton <remy.horton@intel.com>