fix #866: SOCKS5 nil ptr TLSConfig in selector

Move SetDefaultTLSConfig before register(cfg) in loader.go so
ParseService can access it when building handlers. Also add a
defensive nil check in socks5 serverSelector.Select() for
MethodTLS to prevent future ordering regressions.

Root cause: in selector.go, Select() tried to read ClientHello
via TLSConfig.GetConfigForClient but s.TLSConfig was nil because
register(cfg) -> ParseService -> handler Init ran before
SetDefaultTLSConfig was called from loader.go.

handler/socks/v5/selector.go

@@ -29,7 +29,7 @@ func (s *serverSelector) Select(methods ...uint8) (method uint8) {
 	s.logger.Debugf("%d %d %v", gosocks5.Ver5, len(methods), methods)
 	method = gosocks5.MethodNoAuth
 	for _, m := range methods {
-		if m == socks.MethodTLS && !s.noTLS {
+		if m == socks.MethodTLS && !s.noTLS && s.TLSConfig != nil {
 			method = m
 			break
 		}