segfault in ffi_data_to_code_pointer

[DJ Delorie <dj@redhat.com>]

In src/closures.c, ffi_data_to_code_pointer() calls segment_holding()
to get a pointer to the code segment for a data segment.  It doesn't
check for a NULL return, and I've got a test case where I run Ruby's
test suite (on a non-selinux aarch64 machine, if that matters) and
segment_holding() returns NULL and much hilarity ensues.

The following patch fixes the segfault, but I don't know if
segment_holding() returning NULL is an expected case, or a symptom of
problems elsewhere?

> diff -rup a/src/closures.c b/src/closures.c
> --- a/src/closures.c	2019-06-25 21:21:06.738743440 -0400
> +++ b/src/closures.c	2019-06-25 21:22:00.769716129 -0400
> @@ -621,7 +621,10 @@ void *
>  ffi_data_to_code_pointer (void *data)
>  {
>    msegmentptr seg = segment_holding (gm, data);
> -  return add_segment_exec_offset (data, seg);
> +  if (seg)
> +    return add_segment_exec_offset (data, seg);
> +  else
> +    return data;
>  }