ASoC: SOF: fix an Oops, caused by invalid topology

It is possible to create invalid topology that will cause a kernel
Oops when trying to allocate buffers for a NULL substream.
Specifically such an Oops was caused by a topology, where a DAI on a
capture pipeline was referencing the PCM ID from a playback pipeline.
Fix the Oops by explicitly checking for NULL.

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20200124213625.30186-2-pierre-louis.bossart@linux.intel.com


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