[backtrace] Avoid segfault

[Tom de Vries]

On 25-01-19 18:15, Nathan Sidwell wrote:
> On 1/25/19 5:28 AM, Tom de Vries wrote:
>>
>> This patch fixes it by passing "" instead of NULL, in the call to
>> elf_add at line 3083 (for .gnu_debugaltlink), not the call to elf_add at
>> line 3044 (for .gnu_debuglink) mentioned above.
>>
>> Nathan, does this fix the problem for you? If not, can you provide a
>> reproducer, or give a hint on how one could be constructed?
> 
> I still hit the problem, and am installing this as sufficiently obvious.
>  I'm on a fedora system debugging pr88995.  The debuglink_name is
> "../../.dwz/isl-0.16.1-7.fc29.x86_64"
> 

I've managed to reproduce this segfault instance by adding a test-case
that uses both build-id and dwz.

OK for trunk?

Thanks,
- Tom
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