[C++] Add -fnull-this-pointer

[Jan Hubicka]

> On Jan 19, 2016, at 6:41 AM, Jakub Jelinek <jakub@redhat.com> wrote:
> > But then perhaps it will be better incentive for the projects to fix their
> > cruft.  With a specialized option they will keep broken code forever.
> 
> Flags are forever.

OK, in https://gcc.gnu.org/ml/gcc-patches/2016-01/msg01435.html I included the
recomendation of https://gcc.gnu.org/ml/gcc-patches/2016-01/msg01435.html
and https://gcc.gnu.org/ml/gcc-patches/2016-01/msg01435.html to handle the issue.

Honza