libcontainer/configs/config: Clear hook environ variables on empty Env

The runtime spec has [1]:

  * env (array of strings, OPTIONAL) with the same semantics as IEEE
    Std 1003.1-2008's environ.

And running execle or similar with NULL env results in an empty
environent:

  $ cat test.c
  #include <unistd.h>

  int main()
  {
    return execle("/usr/bin/env", "env", NULL, NULL);
  }
  $ cc -o test test.c
  $ ./test
  ...no output...

Go's Cmd.Env, on the other hand, has [2]:

  If Env is nil, the new process uses the current process's
  environment.

This commit works around that by setting Env to an empty slice in
those cases to avoid leaking the runtime environment into the hooks.

[1]: https://github.com/opencontainers/runtime-spec/blob/v1.0.1/config.md#posix-platform-hooks
[2]: https://golang.org/pkg/os/exec/#Cmd

Signed-off-by: W. Trevor King <wking@tremily.us>

libcontainer/configs/config.go

@@ -480,6 +480,9 @@ func (c Command) Run(s *specs.State) error {
 		Stdout: &stdout,
 		Stderr: &stderr,
 	}
+	if cmd.Env == nil {
+		cmd.Env = []string{}
+	}
 	if err := cmd.Start(); err != nil {
 		return err
 	}