qlcnic: fix unchecked return value

Function pci_find_ext_capability() may return 0, which is an invalid
address. In function qlcnic_sriov_virtid_fn(), its return value is used
without validation. This may result in invalid memory access bugs. This
patch fixes the bug.
Signed-off-by: NPan Bian <bianpan2016@163.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/qlogic/qlcnic/qlcnic_sriov_common.c

@@ -128,6 +128,8 @@ static int qlcnic_sriov_virtid_fn(struct qlcnic_adapter *adapter, int vf_id)
 		return 0;
 
 	pos = pci_find_ext_capability(dev, PCI_EXT_CAP_ID_SRIOV);
+	if (!pos)
+		return 0;
 	pci_read_config_word(dev, pos + PCI_SRIOV_VF_OFFSET, &offset);
 	pci_read_config_word(dev, pos + PCI_SRIOV_VF_STRIDE, &stride);