USB: serial: console: fix use-after-free on disconnect

A clean-up patch removing two redundant NULL-checks from the console
disconnect handler inadvertently also removed a third check. This could
lead to the struct usb_serial being prematurely freed by the console
code when a driver accepts but does not register any ports for an
interface which also lacks endpoint descriptors.

Fixes: 0e517c93dc02 ("USB: serial: console: clean up sanity checks")
Cc: stable <stable@vger.kernel.org>     # 4.11
Reported-by: Andrey Konovalov <andreyknvl@google.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>