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

In uhci_handle_td(), the call to ehci_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().

Signed-off-by: Liam Merwick <liam.merwick@oracle.com>
Message-id: 1549460216-25808-7-git-send-email-liam.merwick@oracle.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

hw/usb/hcd-uhci.c

@@ -858,13 +858,15 @@ static int uhci_handle_td(UHCIState *s, UHCIQueue *q, uint32_t qh_addr,
 
     /* Allocate new packet */
     if (q == NULL) {
-        USBDevice *dev = uhci_find_device(s, (td->token >> 8) & 0x7f);
-        USBEndpoint *ep = usb_ep_get(dev, pid, (td->token >> 15) & 0xf);
+        USBDevice *dev;
+        USBEndpoint *ep;
 
-        if (ep == NULL) {
+        dev = uhci_find_device(s, (td->token >> 8) & 0x7f);
+        if (dev == NULL) {
             return uhci_handle_td_error(s, td, td_addr, USB_RET_NODEV,
                                         int_mask);
         }
+        ep = usb_ep_get(dev, pid, (td->token >> 15) & 0xf);
         q = uhci_queue_new(s, qh_addr, td, ep);
     }
     async = uhci_async_alloc(q, td_addr);