net/iavf: fix core dump when the link is unstable

Physical link instability may cause a core dump because unstable
physical links can result in a large number of link change events. Some
of these events may be captured by vf before vf resources are allocated,
and that will result in a core dump.

This commit will check if vf_res is invalid before dereferencing it.

Fixes: 5e03e31 ("net/iavf: handle virtchnl event message without interrupt")
Cc: stable@dpdk.org

Signed-off-by: Kaiwen Deng <kaiwenx.deng@intel.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>

drivers/net/iavf/iavf_vchnl.c

@@ -255,8 +255,8 @@ iavf_read_msg_from_pf(struct iavf_adapter *adapter, uint16_t buf_len,
 		case VIRTCHNL_EVENT_LINK_CHANGE:
 			vf->link_up =
 				vpe->event_data.link_event.link_status;
-			if (vf->vf_res->vf_cap_flags &
-				VIRTCHNL_VF_CAP_ADV_LINK_SPEED) {
+			if (vf->vf_res != NULL &&
+			    vf->vf_res->vf_cap_flags & VIRTCHNL_VF_CAP_ADV_LINK_SPEED) {
 				vf->link_speed =
 				    vpe->event_data.link_event_adv.link_speed;
 			} else {