msm: cpp: Adding null check for regulator disable

Adding null check before regulator enable and also
resetting regulator variable to NULL after regulator disable.
This will avoid using previously disabled regulator in next
session.

Change-Id: I0e8f2f71152064972128f707653e3a2295e1d408
Signed-off-by: Shilpa Mamidi <shilpam@codeaurora.org>