ALSA: line6: Fix NULL dereference at podhd_disconnect()

ALSA: line6: Fix NULL dereference at podhd_disconnect()

BugLink: http://bugs.launchpad.net/bugs/1724669
commit 54a4b2b45817ea2365b40c923c098a26af0c0dbb upstream.

When podhd_init() failed with the acquiring a ctrl i/f, the line6
helper still calls the disconnect callback that eventually calls again
usb_driver_release_interface() with the NULL intf.

Put the proper NULL check before calling it for avoiding an Oops.

Fixes: fc90172ba283 ("ALSA: line6: Claim pod x3 usb data interface")
Reported-by: Andrey Konovalov <andreyknvl@google.com>
Tested-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>