usb: check device is not NULL before calling usb_ep_get()

In musb_packet(), the call to usb_find_device() can return NULL
if it doesn't find a device matching 'addr' so explicitly check
the return value before passing it to usb_ep_get().  This then
allows the subsequent calculation of 'id' to be streamlined.
Signed-off-by: NLiam Merwick <liam.merwick@oracle.com>
Message-id: 1549460216-25808-8-git-send-email-liam.merwick@oracle.com
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

hw/usb/hcd-musb.c

@@ -628,11 +628,11 @@ static void musb_packet(MUSBState *s, MUSBEndPoint *ep,
 
     /* A wild guess on the FADDR semantics... */
     dev = usb_find_device(&s->port, ep->faddr[idx]);
-    uep = usb_ep_get(dev, pid, ep->type[idx] & 0xf);
-    id = pid;
-    if (uep) {
-        id |= (dev->addr << 16) | (uep->nr << 8);
+    if (dev == NULL) {
+        return;
     }
+    uep = usb_ep_get(dev, pid, ep->type[idx] & 0xf);
+    id = pid | (dev->addr << 16) | (uep->nr << 8);
     usb_packet_setup(&ep->packey[dir].p, pid, uep, 0, id, false, true);
     usb_packet_addbuf(&ep->packey[dir].p, ep->buf[idx], len);
     ep->packey[dir].ep = ep;