clk: Don't dereference parent clock if is NULL

The clock passed as an argument to clk_mux_determine_rate_flags()
has the CLK_SET_RATE_PARENT flag set but it has no parent, then a
NULL pointer will tried to be dereferenced.

This shouldn't happen since setting that flag for a clock with no
parent is a bug but the core should be robust to handle that case.

Fixes: 035a61c3 ("clk: Make clk API return per-user struct clk instances")
Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Michael Turquette <mturquette@linaro.org>

drivers/clk/clk.c

@@ -800,8 +800,8 @@ clk_mux_determine_rate_flags(struct clk_hw *hw, unsigned long rate,
 	if (core->flags & CLK_SET_RATE_NO_REPARENT) {
 		parent = core->parent;
 		if (core->flags & CLK_SET_RATE_PARENT)
-			best = __clk_determine_rate(parent->hw, rate,
-						    min_rate, max_rate);
+			best = __clk_determine_rate(parent ? parent->hw : NULL,
+						    rate, min_rate, max_rate);
 		else if (parent)
 			best = clk_core_get_rate_nolock(parent);
 		else