Mail: initialize the "signature" field of ngx_mail_session_t.

Currently it isn't used, but it can be suitable to distinguish
objects stored in c->data.

src/mail/ngx_mail_handler.c

@@ -121,6 +121,8 @@ ngx_mail_init_connection(ngx_connection_t *c)
         return;
     }
 
+    s->signature = NGX_MAIL_MODULE;
+
     s->main_conf = addr_conf->ctx->main_conf;
     s->srv_conf = addr_conf->ctx->srv_conf;