Fix ossl_shim SNI handling

To start with, actually set an SNI callback (copied from bssl_shim); we
weren't actually testing much otherwise (and just happened to have been
passing due to buggy libssl behavior prior to
commit 1c4aa31d

).

Also use proper C++ code for handling C strings -- when a C API
(SSL_get_servername()) returns NULL instead of a string, special-case
that instead of blindly trying to compare NULL against a std::string,
and perform the comparsion using the std::string operators instead of
falling back to pointer comparison.

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6792)