Fix BZ #18084 (backtrace(..., 0) crash on x86)

[Tolga Dalman]

On 08/10/2015 02:04 AM, Paul Pluzhnikov wrote:
> This patch fixes BZ #18084 -- crash in backtrace(..., 0) on x86.
> Tested on Linux/x86, no new failures.

That looks correct and should be applied to the other arches as well
(I looked at x86_64 and arm).

Best regards
Tolga Dalman