libnetwork/pasta: fix possible panic in error cast

exec.Command().Output() can fail for several errors, i.e. maybe it was
not able to execute at all. We only get a error from type
`exec.ExitError` when the command exited > 0. For all other cases this
cast was unsafe and would cause a panic with a nil deref.

First check the error type correctly with errors.As() then return the
stderr and exit code. Otherwise just return the wrapped error as is.

Signed-off-by: Paul Holzinger <pholzing@redhat.com>

common/libnetwork/pasta/pasta.go

@@ -11,6 +11,7 @@
 package pasta
 
 import (
+	"errors"
 	"fmt"
 	"os/exec"
 	"strings"
@@ -124,8 +125,12 @@ func Setup(opts *SetupOptions) error {
 	// pasta forks once ready, and quits once we delete the target namespace
 	_, err = exec.Command(path, cmdArgs...).Output()
 	if err != nil {
-		return fmt.Errorf("failed to start pasta:\n%s",
-			err.(*exec.ExitError).Stderr)
+		exitErr := &exec.ExitError{}
+		if errors.As(err, &exitErr) {
+			return fmt.Errorf("pasta failed with exit code %d:\n%s",
+				exitErr.ExitCode(), exitErr.Stderr)
+		}
+		return fmt.Errorf("failed to start pasta: %w", err)
 	}
 
 	return nil