alsa-lib: fixed coverity reported issues under "FORWARD_NULL" checker.

Coverity Static Analysis helps developers find hard-to-spot,
yet potentially crash-causing defects early in the development phase,
reducing the cost,time, and risk of software errors.

This patch has fix for situations where variable can be NULL
but not been checked beforehand

Signed-off-by: Takashi Iwai <tiwai@suse.de>
7 files changed