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.comSigned-off-by: Mark Brown <broonie@kernel.org>