crypto: mv_cesa - ensure backlog is initialised

backlog is not initialised so in the case where
cpg->eng_st != ENGINE_IDLE it is never initialised and
hence which could lead to an illegal memory dereference
in the statement:

  backlog->complete(backlog, -EINPROGRESS);

Discovered with cppcheck static analsys:
[drivers/crypto/mv_cesa.c:616]:
   (error) Uninitialized variable: backlog
Signed-off-by: NColin Ian King <colin.king@canonical.com>
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>

drivers/crypto/mv_cesa.c

@@ -595,7 +595,7 @@ static int queue_manag(void *data)
 	cpg->eng_st = ENGINE_IDLE;
 	do {
 		struct crypto_async_request *async_req = NULL;
-		struct crypto_async_request *backlog;
+		struct crypto_async_request *backlog = NULL;
 
 		__set_current_state(TASK_INTERRUPTIBLE);