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

[Roland McGrath]

> 2015-08-15  Paul Pluzhnikov  <ppluzhnikov@google.com>
> 
	  [BZ #18084]
>         * debug/tst-backtrace2.c (do_test): Add test for BZ #18084.
>         * sysdeps/arm/backtrace.c (__backtrace): Handle size == 0.
>         * sysdeps/i386/backtrace.c (__backtrace): Likewise.
>         * sysdeps/m68k/backtrace.c (__backtrace): Likewise.
>         * sysdeps/microblaze/backtrace.c (__backtrace): Likewise.
>         * sysdeps/s390/s390-32/backtrace.c (__backtrace): Likewise.
>         * sysdeps/s390/s390-64/backtrace.c (__backtrace): Likewise.
>         * sysdeps/sparc/backtrace.c (__backtrace): Likewise.
>         * sysdeps/x86_64/backtrace.c (__backtrace): Likewise.