ASoC: Fix NULL dereference at freeing

When an ASoC driver with pcm_destruct component ops is freed before
the PCM object instantiation (e.g. deferring the probe), it hits an
Oops at snd_soc_pcm_component_free() that calls the pcm_destruct ops
unconditionally.

Fix it by adding a NULL-check of rtd->pcm before calling callbacks.

Fixes: c64bfc90 ("ASoC: soc-core: add new pcm_construct/pcm_destruct")
Reported-by: Matthias Brugger <matthias.bgg@gmail.com>
Tested-by: Ben Ho <ben.ho@mediatek.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://lore.kernel.org/r/20200107070956.15807-1-tiwai@suse.deSigned-off-by: Mark Brown <broonie@kernel.org>

sound/soc/soc-component.c

@@ -520,6 +520,9 @@ void snd_soc_pcm_component_free(struct snd_soc_pcm_runtime *rtd)
 	struct snd_soc_rtdcom_list *rtdcom;
 	struct snd_soc_component *component;
 
+	if (!rtd->pcm)
+		return;
+
 	for_each_rtd_components(rtd, rtdcom, component)
 		if (component->driver->pcm_destruct)
 			component->driver->pcm_destruct(component, rtd->pcm);