SSL: detect "listen ... ssl" without certificates (ticket #178). In mail and stream modules, no certificate provided is a fatal condition, much like with the "ssl" and "starttls" directives. In http, "listen ... ssl" can be used in a non-default server without certificates as long as there is a certificate in the default one, so missing certificate is only fatal for default servers.

--- a/src/mail/ngx_mail_ssl_module.h
+++ b/src/mail/ngx_mail_ssl_module.h
@@ -26,6 +26,7 @@ typedef struct {
     ngx_ssl_t        ssl;
 
     ngx_uint_t       starttls;
+    ngx_uint_t       listen;
     ngx_uint_t       protocols;
 
     ngx_uint_t       verify;