RDMA/amso1100: Add check if cache memory was allocated before freeing it

There is a path in handle_vq() where kmem_cache_free() can be called
with pointer to a local variable.  It can happen if vq_repbuf_alloc()
failed to allocate memory from cache and req is NULL.

The patch adds check if cache memory was allocated before freeing it.

Found by Linux Driver Verification project (linuxtesting.org).
Signed-off-by: NSvetlana Mavrina <another.karnil@gmail.com>
Reviewed-by: NAlexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: NRoland Dreier <roland@purestorage.com>

drivers/infiniband/hw/amso1100/c2_intr.c

@@ -169,7 +169,8 @@ static void handle_vq(struct c2_dev *c2dev, u32 mq_index)
 		 * We should never get here, as the adapter should
 		 * never send us a reply that we're not expecting.
 		 */
-		vq_repbuf_free(c2dev, host_msg);
+		if (reply_msg != NULL)
+			vq_repbuf_free(c2dev, host_msg);
 		pr_debug("handle_vq: UNEXPECTEDLY got NULL req\n");
 		return;
 	}