ASoC: wm_adsp: Avoid narrow race condition on compr pointer

There is a very small window between then wm_adsp_compr_free gets call
and when the DSP is actually powered down. If we get an IRQ from the DSP
in this window then the wm_adsp_compr pointer will be NULL. This patch
adds a check for this into the IRQ handler to avoid any issues when this
happens.
Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

sound/soc/codecs/wm_adsp.c

@@ -2845,7 +2845,7 @@ int wm_adsp_compr_handle_irq(struct wm_adsp *dsp)
 		goto out;
 	}
 
-	if (compr->stream)
+	if (compr && compr->stream)
 		snd_compr_fragment_elapsed(compr->stream);
 
 out: