ASoC: codecs: add null check before use

Add null check for wcd938x component and wcd938x_priv
before use.
Add array bounds check for slave channel index before
accessing the array.

Change-Id: Ida6b2044f5f9b9a65f4b45d868040af8bd2116eb
Signed-off-by: Vignesh Kulothungan <vigneshk@codeaurora.org>