nss: introduce signed integer overflow check in malloc

[Florian Weimer]

* Marcus Poller:

> Thank you Arjun and Andreas for the detailed feedback, I have taken it 
> into account and would like to submit revision 2:
>
> nss: Use reallocarray to prevent integer overflow in getaddrinfo (bug 33977)
>
> replacing realloc by reallocarray introduces a basic overflow check.
> (old + count) might still overflow, but since the NSS backend is trusted,
> we do not consider this to be a valid case.
> ---
>   nss/getaddrinfo.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/nss/getaddrinfo.c b/nss/getaddrinfo.c
> index 4f6ac3358a..b6ac0b2dcc 100644
> --- a/nss/getaddrinfo.c
> +++ b/nss/getaddrinfo.c
> @@ -234,7 +234,7 @@ convert_hostent_to_gaih_addrtuple (const struct 
> addrinfo *req, int family,
>         array = array->next;
>       }
>
> -  array = realloc (res->at, (old + count) * sizeof (*array));
> +  array = reallocarray (res->at, old + count, sizeof (*array));
>
>     if (array == NULL)
>       return false;

The subject says “signed integer overflow”, but as far as I can see,
this is size_t wraparound.

Thanks,
Florian