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

libcontainer/container_linux.go

@@ -531,7 +531,7 @@ func (c *Container) newParentProcess(p *Process) (parentProcess, error) {
 				logrus.Debug("runc-dmz: using runc-dmz") // used for tests
 			} else if errors.Is(err, dmz.ErrNoDmzBinary) {
 				logrus.Debug("runc-dmz binary not embedded in runc binary, falling back to /proc/self/exe clone")
-			} else if err != nil {
+			} else {
 				return nil, fmt.Errorf("failed to create runc-dmz binary clone: %w", err)
 			}
 		} else {