[ALSA] soc - 0.13 ASoC DAPM bug fix for unnamed streams

This patch fixes a bug whereby an unnamed stream would cause a NULL
pointer ref in snd_soc_dapm_stream_event().

Signed-off-by: Seth Forshee <seth.forshee@gmail.com>
Signed-off-by: Liam Girdwood <lg@opensource.wolfsonmicro.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>