RDMA/qedr: add null check before pointer dereference

Add null check before dereferencing pointer sgid_attr.ndev
inside function rdma_vlan_dev_vlan_id().

Addresses-Coverity-ID: 1373979
Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Acked-by: Ram Amrani <Ram.Amrani@cavium.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>