clk: make sure parent is not NULL in clk_fetch_parent_index()

If parent is given with NULL, clk_fetch_parent_index() could return
a positive index value.

Currently, parent is checked by the callers of this function, but
it would be safer to do it in this function.
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: NVladimir Zapolskiy <vz@mleia.com>
Signed-off-by: NStephen Boyd <sboyd@codeaurora.org>

drivers/clk/clk.c

@@ -1066,6 +1066,9 @@ static int clk_fetch_parent_index(struct clk_core *core,
 {
 	int i;
 
+	if (!parent)
+		return -EINVAL;
+
 	/*
 	 * find index of new parent clock using cached parent ptrs,
 	 * or if not yet cached, use string name comparison and cache