[ALSA] sound: initialize rawmidi substream list

If snd_rawmidi_new() failed to allocate substreams for input
(snd_rawmidi_alloc_substreams() failed to populate a
&rmidi->streams[SNDRV_RAWMIDI_STREAM_INPUT]), it will try to
free rawmidi instance by snd_rawmidi_free().
But it will cause oops because snd_rawmidi_free() tries to free
both of substreams list but list for output
(&rmidi->streams[SNDRV_RAWMIDI_STREAM_OUTPUT]) is not initialized yet.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>