ssl: graceful handling of bad SNI configuration. (#3230)

Previously, we would assert when we failed to set SNI for a socket. Now,
we reject the bad config.

Risk Level: Low
Testing: New ssl_socket_test.

Signed-off-by: Harvey Tuch <htuch@google.com>

Mirrored from https://github.com/envoyproxy/envoy @ 3b084a7d747750cfcb868f0cce463af2fe4e781c

envoy/api/v2/auth/cert.proto

@@ -215,7 +215,7 @@ message UpstreamTlsContext {
   CommonTlsContext common_tls_context = 1;
 
   // SNI string to use when creating TLS backend connections.
-  string sni = 2;
+  string sni = 2 [(validate.rules).string.max_bytes = 255];
 }
 
 message DownstreamTlsContext {