s390/cio: add NULL test

Add NULL test on call to kzalloc.

The semantic match that finds this problem is as follows:
(http://coccinelle.lip6.fr/)

// <smpl>
@@
expression x;
identifier fld;
@@

* x = kzalloc(...);
  ... when != x == NULL
  x->fld
// </smpl>
Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

drivers/s390/char/con3215.c

@@ -922,6 +922,8 @@ static int __init con3215_init(void)
 	spin_lock_init(&raw3215_freelist_lock);
 	for (i = 0; i < NR_3215_REQ; i++) {
 		req = kzalloc(sizeof(struct raw3215_req), GFP_KERNEL | GFP_DMA);
+		if (!req)
+			return -ENOMEM;
 		req->next = raw3215_freelist;
 		raw3215_freelist = req;
 	}