clk: fix possible null pointer dereference

The commit 646cafc6 (clk: Change clk_ops->determine_rate to
return a clk_hw as the best parent) opens a possibility for
null pointer dereference, fix this.

Signed-off-by: Stanimir Varbanov <svarbanov@mm-sol.com>
Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Michael Turquette <mturquette@linaro.org>
diff --git a/drivers/clk/clk.c b/drivers/clk/clk.c
index f4963b7..d48ac71 100644
--- a/drivers/clk/clk.c
+++ b/drivers/clk/clk.c
@@ -1366,7 +1366,7 @@
 		new_rate = clk->ops->determine_rate(clk->hw, rate,
 						    &best_parent_rate,
 						    &parent_hw);
-		parent = parent_hw->clk;
+		parent = parent_hw ? parent_hw->clk : NULL;
 	} else if (clk->ops->round_rate) {
 		new_rate = clk->ops->round_rate(clk->hw, rate,
 						&best_parent_rate);