ASoC: SOF: topology: fix missing NULL pointer check

Add check to avoid possible NULL pointer dereference issue.

This issue was reported by static analysis tools, we didn't face this
issue but we can't rule it out either as a false positive.

Reported-by: Keqiao Zhang <keqiao.zhang@intel.com>
Signed-off-by: Keyon Jie <yang.jie@linux.intel.com>
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20191101170916.26517-2-pierre-louis.bossart@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>