ASoC: Check for a CODEC before dereferencing in DAPM

A CODEC pointer is optional (and is checked for in most contexts within
DAPM) - add checks to the few places where it was missed.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>