tcptls: NULL-check the parameter of ast_ssl_teardown before accessing it.

This avoids a crash on stopping a chan_sip which failed to start its TLS server.

ASTERISK-27339 #close

Change-Id: I327fc70db68eaaca5b50a15c7fd687fde79263d5

main/tcptls.c

@@ -500,7 +500,7 @@ int ast_ssl_setup(struct ast_tls_config *cfg)
 void ast_ssl_teardown(struct ast_tls_config *cfg)
 {
 #ifdef DO_SSL
-	if (cfg->ssl_ctx) {
+	if (cfg && cfg->ssl_ctx) {
 		SSL_CTX_free(cfg->ssl_ctx);
 		cfg->ssl_ctx = NULL;
 	}