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 <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)