Re: [PATCH] warn on overflow in calls to allocation functions (bugs 77531 and 78284)

[Martin Sebor]

+enabled with @option{-Wextra}.

So I think we should in the immediate term not enable this in Wextra.
However, I think for gcc-8 we should revisit after fixing GCC to be
cleaner WRT alloc-zero.

So disable alloc-zero by default, comment typo and potentially adding
the GTY marker to alloc_object_size_limit and this is OK.

Thanks.  Committed in r243470 with the changes above.

Martin