s_client: avoid warning on Windows/MS-DOS systems

it appears when using gcc/mingw:
```
apps/s_client.c:815:9: warning: variable 'at_eof' set but not used [-Wunused-but-set-variable]
     int at_eof = 0;
         ^~~~~~
```
Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1512)