ASoC: fsl_easrc: Check for null pointer before dereferencing "ctx" in fsl_easrc_hw_free()

The patch 955ac624: "ASoC: fsl_easrc: Add EASRC ASoC CPU DAI
drivers" from Apr 16, 2020, leads to the following Smatch complaint:

sound/soc/fsl/fsl_easrc.c:1529 fsl_easrc_hw_free()
warn: variable dereferenced before check 'ctx' (see line 1527)

sound/soc/fsl/fsl_easrc.c
  1526          struct fsl_asrc_pair *ctx = runtime->private_data;
  1527          struct fsl_easrc_ctx_priv *ctx_priv = ctx->private;
                                                      ^^^^^
Dereference

  1528
  1529          if (ctx && (ctx_priv->ctx_streams & BIT(substream->stream))) {
                    ^^^
This check is too late, to prevent a NULL dereference.

  1530                  ctx_priv->ctx_streams &= ~BIT(substream->stream);
  1531                  fsl_easrc_release_context(ctx);

Fixes: 955ac624 ("ASoC: fsl_easrc: Add EASRC ASoC CPU DAI drivers")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Shengjiu Wang <shengjiu.wang@nxp.com>
Acked-by: Nicolin Chen <nicoleotsuka@gmail.com>
Link: https://lore.kernel.org/r/d23c939f1c9eeb3fce34b6c34d44e2d6156f663a.1587799355.git.shengjiu.wang@nxp.comSigned-off-by: Mark Brown <broonie@kernel.org>

sound/soc/fsl/fsl_easrc.c

@@ -1524,9 +1524,14 @@ static int fsl_easrc_hw_free(struct snd_pcm_substream *substream,
 {
 	struct snd_pcm_runtime *runtime = substream->runtime;
 	struct fsl_asrc_pair *ctx = runtime->private_data;
-	struct fsl_easrc_ctx_priv *ctx_priv = ctx->private;
+	struct fsl_easrc_ctx_priv *ctx_priv;
+
+	if (!ctx)
+		return -EINVAL;
+
+	ctx_priv = ctx->private;
 
-	if (ctx && (ctx_priv->ctx_streams & BIT(substream->stream))) {
+	if (ctx_priv->ctx_streams & BIT(substream->stream)) {
 		ctx_priv->ctx_streams &= ~BIT(substream->stream);
 		fsl_easrc_release_context(ctx);
 	}