usb-desc: fix user trigerrable segfaults (!config)

Check for dev->config being NULL in two places:
 USB_REQ_GET_CONFIGURATION and USB_REQ_GET_STATUS.

The behavior of USB_REQ_GET_STATUS is unspecified in the Default state,
that corresponds to dev->config being NULL (it defaults to NULL and is
reset whenever a SET_CONFIGURATION with value 0, or attachment). I
implemented it to correspond with the state before
ed5a83ddd8c1d8ec7b1015315530cf29949e7c48, the commit moving SET_STATUS
to usb-desc; if dev->config is not set we return whatever is in the
first configuration.

The behavior of USB_REQ_GET_CONFIGURATION is also undefined before any
SET_CONFIGURATION, but here we just return 0 (same as specified for the
Address state).

A win7 guest failed to initialize the device before this patch,
segfaulting when GET_STATUS was called with dev->config == NULL. With
this patch the passthrough device still doesn't work but the failure is
unrelated.

Signed-off-by: Alon Levy <alevy@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
