txt_db: fix -Wunused-but-set-variable
txt_db: fix -Wunused-but-set-variable

The loop never uses the value of 'ln'.

Fixes this error with Clang 15:
```
crypto/txt_db/txt_db.c:24:10: error: variable 'ln' set but not used [-Werror,-Wunused-but-set-variable]
    long ln = 0;
         ^
1 error generated.
```

Signed-off-by: Sam James <sam@gentoo.org>

Reviewed-by: Paul Dale <ddec78389666b4f7c9b4d110380c848489a3e4aa@openssl.org>
Reviewed-by: Hugo Landau <ce9fa1e55a72fc9b85dd06e47ea2c07848fb7d2c@openssl.org>
Reviewed-by: Tomas Mraz <2bc6038c3dfca09b2da23c8b6da8ba884dc2dcc2@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/19450)

openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl