usb: check USB configuration descriptor object

When processing remote NDIS control message packets, the USB Net
device emulator checks to see if the USB configuration descriptor
object is of RNDIS type(2). But it does not check if it is null,
which leads to a null dereference error. Add check to avoid it.
Reported-by: NQinghao Tang <luodalongde@gmail.com>
Signed-off-by: NPrasad J Pandit <pjp@fedoraproject.org>
Message-id: 1455188480-14688-1-git-send-email-ppandit@redhat.com
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

hw/usb/dev-network.c

@@ -654,7 +654,8 @@ typedef struct USBNetState {
 
 static int is_rndis(USBNetState *s)
 {
-    return s->dev.config->bConfigurationValue == DEV_RNDIS_CONFIG_VALUE;
+    return s->dev.config ?
+            s->dev.config->bConfigurationValue == DEV_RNDIS_CONFIG_VALUE : 0;
 }
 
 static int ndis_query(USBNetState *s, uint32_t oid,