Dear all,

we did not check array element triplets for validity strictly enough
(i.e. defensively in the case of invalid code), so we could encounter
non-integer constant expressions that were passed to mpz_get_si.

The attached obvious patch tries to fix all such potential issues
in get_expr_storage_size.

Regtested on x86_64-pc-linux-gnu.  OK for mainline?

Thanks,
Harald