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 <[e-mail]>
Reviewed-by: Stephen Boyd <[e-mail]>
Signed-off-by: Michael Turquette <[e-mail]>

drivers/clk/clk.c

···
 		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);