usb: gadget: fix null pointer dereference in usb_put_function_instance()

In the Linux kernel, the following vulnerability has been resolved: usb: gadget: fix null pointer dereference in usb_put_function_instance() usb_put_function_instance() attempts to dereference fd inside fi struct to get mod in uvc_alloc_inst() error path. However, fd is not allocated until later in try_get_usb_function_instance() after allocating fi in uvc_alloc_inst() and thus guranteed to be null in error path. Fix this by adding a null check for fi->fd that returns if fd is null.