rootless: do not segfault if the parent already died

Closes: https://github.com/projectatomic/libpod/issues/1189

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

pkg/rootless/rootless_linux.c

@@ -97,6 +97,8 @@ reexec_in_user_namespace(int ready)
     return pid;
 
   argv = get_cmd_line_args (ppid);
+  if (argv == NULL)
+    _exit (EXIT_FAILURE);
 
   setenv ("_LIBPOD_USERNS_CONFIGURED", "init", 1);
   setenv ("_LIBPOD_ROOTLESS_UID", uid, 1);