Fix arithmetic expression overflow

Fix arithmetic expression overflow

If the value of a->length is large (>= 2^12), then an integer overflow will
occur for the signed type, which according to the C standard is UB.

CLA: trivial

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/23274)

(cherry picked from commit 486ab0fb003d05f89620662260486d31bd3faa8c)