strnlen_user() returns 0 when it hits fault, not -1. Fix the test in
audit_log_single_execve_arg(). Luckily this shouldn't ever happen unless
there's a kernel bug so it's mostly a cosmetic fix.
CC: Paul Moore <pmoore@redhat.com> Signed-off-by: Jan Kara <jack@suse.cz> Signed-off-by: Paul Moore <pmoore@redhat.com>