clk: qcom: clk-alpha-pll: check for null when trying to get parent

In clk_fabia_pll_prepare, check for null when trying to get parent
from clk_hw before attempting to dereference.

Change-Id: I95ea1dea818b534fb9e92950a4fe72b77d23896c
Signed-off-by: David Dai <daidavid1@codeaurora.org>