ALSA: Handle NULL jacks in snd_jack_report()

Facilitate drivers that wish to carry on if they can't create a jack
input device by handling attempts to report the state of a NULL jack,
removing the need to check for initialisation before use.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>