Use INVALID_SOCKET in SSLEcho demo

Use INVALID_SOCKET in SSLEcho demo

On Windows, SOCKET type is unsigned.

All comparison with negative value produces signed/unsigned
warnings, moreover the code is incorrect in error path.

Use INVALID_SOCKET define that should work on all
platforms to detect error.

Reviewed-by: Tomas Mraz <tomas@openssl.foundation>
Reviewed-by: Eugene Syromiatnikov <esyr@openssl.org>
Reviewed-by: Norbert Pocs <norbertp@openssl.org>
MergeDate: Thu Apr 30 11:42:31 2026
(Merged from https://github.com/openssl/openssl/pull/30941)