Fix warning in setjmp/jmpbug.c

[Ondřej Bílka]

On Wed, Nov 26, 2014 at 12:37:49AM +0000, Joseph Myers wrote:
> This patch fixes a "set but not used" warning in setjmp/jmpbug.c.  A
> variable is used only to store the result of alloca.  A cast to void
> is added to avoid the warning, and the variable is made volatile to
> ensure the call to alloca isn't optimized away for being unused.
> 
> Tested for x86_64.
> 
looks ok.