ASoC: Intel: Skylake: Fix potential null pointer dereference

Check if the next sink is not null to avoid potential null pointer
dereference in skl_tplg_bind_sinks().

Signed-off-by: Guneshwor Singh <guneshwor.o.singh@intel.com>
Acked-By: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>