Fix CVE-2017-15132:

https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2017-15132

Patch copied from upstream source repository:

https://github.com/dovecot/core/commit/1a29ed2f96da1be22fa5a4d96c7583aa81b8b060

From 1a29ed2f96da1be22fa5a4d96c7583aa81b8b060 Mon Sep 17 00:00:00 2001
From: Timo Sirainen <timo.sirainen@dovecot.fi>
Date: Mon, 18 Dec 2017 16:50:51 +0200
Subject: [PATCH] lib-auth: Fix memory leak in auth_client_request_abort()

This caused memory leaks when authentication was aborted. For example
with IMAP:

a AUTHENTICATE PLAIN
*

Broken by 9137c55411aa39d41c1e705ddc34d5bd26c65021
---
 src/lib-auth/auth-client-request.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/src/lib-auth/auth-client-request.c b/src/lib-auth/auth-client-request.c
index 480fb42b30..046f7c307d 100644
--- a/src/lib-auth/auth-client-request.c
+++ b/src/lib-auth/auth-client-request.c
@@ -186,6 +186,7 @@ void auth_client_request_abort(struct auth_client_request **_request)
 
 	auth_client_send_cancel(request->conn->client, request->id);
 	call_callback(request, AUTH_REQUEST_STATUS_ABORT, NULL, NULL);
+	pool_unref(&request->pool);
 }
 
 unsigned int auth_client_request_get_id(struct auth_client_request *request)
