staging: mei: fix possible usage of uninitialized variable

Fix warning: reported in http://marc.info/?l=linux-kernel&m=130812960402606&w=2

drivers/staging/mei/interrupt.c: warning: 'buffer' may be used uninitialized in this function:  => 198

It is a real issue and wrong path in execution is taken when
list is empty or (cl && _mei_irq_thread_state_ok(cl, mei_hdr))
evaluates to false
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/staging/mei/interrupt.c

@@ -195,7 +195,7 @@ static int mei_irq_thread_read_client_message(struct mei_io_list *complete_list,
 {
 	struct mei_cl *cl;
 	struct mei_cl_cb *cb_pos = NULL, *cb_next = NULL;
-	unsigned char *buffer;
+	unsigned char *buffer = NULL;
 
 	dev_dbg(&dev->pdev->dev, "start client msg\n");
 	if (!(dev->read_list.status == 0 &&