pem: fix -Wunused-but-set-variable

pem: fix -Wunused-but-set-variable

The loop never uses the value of 'line'.

Fixes this error with Clang 15:
```
crypto/pem/pem_lib.c:821:14: error: variable 'line' set but not used [-Werror,-Wunused-but-set-variable]
    int len, line, ret = 0, end = 0, prev_partial_line_read = 0, partial_line_read = 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)