usb_find_alt_setting() takes a pointer to a struct usb_host_config as
an argument; it searches for an interface with specified interface and
alternate setting numbers in that config. However, it crashes if the
usb_host_config pointer argument is NULL.
Since this is a general-purpose routine, available for use in many
places, we want to to be more robust. This patch makes it return NULL
whenever the config argument is NULL.