ASoC: imx-audmux: Check for NULL pointer

Check for NULL pointer before accessing it.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
diff --git a/sound/soc/imx/imx-audmux.c b/sound/soc/imx/imx-audmux.c
index 912a342..0fe66c3 100644
--- a/sound/soc/imx/imx-audmux.c
+++ b/sound/soc/imx/imx-audmux.c
@@ -79,6 +79,9 @@
 	if (!buf)
 		return -ENOMEM;
 
+	if (!audmux_base)
+		return -ENOSYS;
+
 	if (audmux_clk)
 		clk_prepare_enable(audmux_clk);