clk: Don't dereference parent clock if is NULL

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: 035a61c314eb3 ("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>