Enable govet nilness, fix an issue

The code already checked if err == nil above, so the linter complains:

> libcontainer/container_linux.go:534:18: nilness: tautological condition: non-nil != nil (govet)
> 			} else if err != nil {
> 			              ^

Fix the issue, enable the check.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

.golangci.yml

@@ -11,3 +11,8 @@ linters:
     - errorlint
     - unconvert
     - unparam
+
+linters-settings:
+  govet:
+    enable:
+      - nilness