[sanitizer] only lower the soft coresize limit in DisableCoreDumper

[Stephan Bergmann]

...so that e.g. a spawned sub-thread could increase that limit again:

> Index: lib/sanitizer_common/sanitizer_posix_libcdep.cc
> ===================================================================
> --- lib/sanitizer_common/sanitizer_posix_libcdep.cc	(revision 211695)
> +++ lib/sanitizer_common/sanitizer_posix_libcdep.cc	(working copy)
> @@ -46,8 +46,10 @@
>
>  void DisableCoreDumper() {
>    struct rlimit nocore;
> +  if (getrlimit(RLIMIT_CORE, &nocore) == -1) {
> +      nocore.rlim_max = 0;
> +  }
>    nocore.rlim_cur = 0;
> -  nocore.rlim_max = 0;
>    setrlimit(RLIMIT_CORE, &nocore);
>  }
>

Stephan