Potential integer overflow in sds.c

The `sdsnewlen` and `sdsMakeRoomFor` function implemented in [sds.c](https://github.com/antirez/sds/blob/master/sds.c) is quite similiar to those in the [redis](https://github.com/redis/redis). Thus, it's very likely that this integer overflow in [CVE-2021-21309](https://nvd.nist.gov/vuln/detail/CVE-2021-21309) also affects [sds](https://github.com/antirez/sds).
Would you can help to check if this bug is true? If it's true, I'd like to open a PR for that if necessary. Thank you for your effort and patience!
And here is the [patch](https://github.com/redis/redis/commit/d32f2e9999ce003bad0bd2c3bca29f64dcce4433) for [CVE-2021-21309](https://nvd.nist.gov/vuln/detail/CVE-2021-21309) for your reference if this issue needs to be fixed.