usb: musb: host: fix potential NULL pointer dereference

musb_start_urb() doesn't check the pass-in parameter if it is NULL.  But
in musb_bulk_nak_timeout() the parameter passed to musb_start_urb() is
returned from first_qh(), which could be NULL.

So wrap the musb_start_urb() call here with a if condition check to
avoid the potential NULL pointer dereference.

Fixes: f283862f ("usb: musb: NAK timeout scheme on bulk TX endpoint")
Cc: stable@vger.kernel.org # v3.7+
Signed-off-by: Bin Liu <b-liu@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/musb/musb_host.c

@@ -990,6 +990,8 @@ static void musb_bulk_nak_timeout(struct musb *musb, struct musb_hw_ep *ep,
 			/* set tx_reinit and schedule the next qh */
 			ep->tx_reinit = 1;
 		}
+
+		if (next_qh)
 			musb_start_urb(musb, is_in, next_qh);
 	}
 }