snd_soc_dapm_new_dai may return an error pointer and currently this
isn't checked for in dapm_connect_dai_link_widgets. Add code to check
the return value and not add routes in that case.

Fixes: 778ff5bb ("ASoC: dapm: Move connection of CODEC to CODEC DAIs")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Signed-off-by: Mark Brown <broonie@kernel.org>