Fix possible memory over-read in apps/s_client.c

a buffer returned from BIO_gets is not checked for it's length before
reading its contents.

Reviewed-by: Ben Kaduk <kaduk@mit.edu>
Reviewed-by: Andy Polyakov <appro@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3630)