x509: fix -Wunused-but-set-variable

x509: fix -Wunused-but-set-variable

The value of 'l' isn't ever actually used.

Fixes this error with Clang 15:
```
crypto/x509/x_name.c:506:9: error: variable 'l' set but not used [-Werror,-Wunused-but-set-variable]
    int l, i;
        ^
1 error generated.
```

Signed-off-by: Sam James <sam@gentoo.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Hugo Landau <hlandau@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/19450)