Mail: fix session cleanup on error path

Previously, when ngx_handle_read_event() or ngx_handle_write_event()
returned an error while handling an SMTP, POP3 or IMAP session, the
released session memory could be accessed after handling the error.

src/mail/ngx_mail_imap_handler.c

@@ -48,6 +48,7 @@ ngx_mail_imap_init_session(ngx_mail_session_t *s, ngx_connection_t *c)
 
     if (ngx_handle_read_event(c->read, 0) != NGX_OK) {
         ngx_mail_close_connection(c);
+        return;
     }
 
     ngx_mail_send(c->write);

src/mail/ngx_mail_pop3_handler.c

@@ -69,6 +69,7 @@ ngx_mail_pop3_init_session(ngx_mail_session_t *s, ngx_connection_t *c)
 
     if (ngx_handle_read_event(c->read, 0) != NGX_OK) {
         ngx_mail_close_connection(c);
+        return;
     }
 
     ngx_mail_send(c->write);

src/mail/ngx_mail_proxy_module.c

@@ -894,6 +894,7 @@ ngx_mail_proxy_write_handler(ngx_event_t *wev)
 
     if (ngx_handle_write_event(wev, 0) != NGX_OK) {
         ngx_mail_proxy_internal_server_error(s);
+        return;
     }
 
     if (c->read->ready) {

src/mail/ngx_mail_smtp_handler.c

@@ -340,6 +340,7 @@ ngx_mail_smtp_greeting(ngx_mail_session_t *s, ngx_connection_t *c)
 
     if (ngx_handle_read_event(c->read, 0) != NGX_OK) {
         ngx_mail_close_connection(c);
+        return;
     }
 
     if (c->read->ready) {