qcom-charger: pmic-voter: destroy only valid votables

Votables that are NULL or have already been destroyed need not be
destroyed again. Return immediately when destorying NULL votables.

CRs-Fixed: 1023141
Change-Id: I8e367dbe7acf72471a5a474f0e2a00a4004fcbfb
Signed-off-by: Nicholas Troast <ntroast@codeaurora.org>