ALSA: firewire: fix NULL pointer dereference when releasing uninitialized data of iso-resource

When calling 'iso_resource_free()' for uninitialized data, this function
causes NULL pointer dereference due to its 'unit' member. This occurs when
unplugging audio and music units on IEEE 1394 bus at failure of card
registration.

This commit fixes the bug. The bug exists since kernel v4.5.

Fixes: 324540c4 ('ALSA: fireface: postpone sound card registration') at v4.12
Fixes: 8865a31e ('ALSA: firewire-motu: postpone sound card registration') at v4.12
Fixes: b610386c ('ALSA: firewire-tascam: deleyed registration of sound card') at v4.7
Fixes: 86c8dd7f ('ALSA: firewire-digi00x: delayed registration of sound card') at v4.7
Fixes: 6c29230e ('ALSA: oxfw: delayed registration of sound card') at v4.7
Fixes: 7d3c1d59 ('ALSA: fireworks: delayed registration of sound card') at v4.7
Fixes: 04a2c73c ('ALSA: bebob: delayed registration of sound card') at v4.7
Fixes: b59fb190 ('ALSA: dice: postpone card registration') at v4.5
Cc: <stable@vger.kernel.org> # v4.5+
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>