clk: Fix clk_core_get NULL dereference

BugLink: https://bugs.launchpad.net/bugs/2060097

[ Upstream commit e97fe4901e0f59a0bfd524578fe3768f8ca42428 ]

It is possible for clk_core_get to dereference a NULL in the following
sequence:

clk_core_get()
    of_clk_get_hw_from_clkspec()
        __of_clk_get_hw_from_provider()
            __clk_get_hw()

__clk_get_hw() can return NULL which is dereferenced by clk_core_get() at
hw->core.

Prior to commit dde4eff47c ("clk: Look for parents with clkdev based
clk_lookups") the check IS_ERR_OR_NULL() was performed which would have
caught the NULL.

Reading the description of this function it talks about returning NULL but
that cannot be so at the moment.

Update the function to check for hw before dereferencing it and return NULL
if hw is NULL.

Fixes: dde4eff47c ("clk: Look for parents with clkdev based clk_lookups")
Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Link: https://lore.kernel.org/r/20240302-linux-next-24-03-01-simple-clock-fixes-v1-1-25f348a5982b@linaro.org
Signed-off-by: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
(cherry picked from commit 6f073b24a9e2becd25ac4505a9780a87e621bb51)
Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>

drivers/clk/clk.c

@@ -418,6 +418,9 @@ static struct clk_core *clk_core_get(struct clk_core *core, u8 p_index)
 	if (IS_ERR(hw))
 		return ERR_CAST(hw);
 
+	if (!hw)
+		return NULL;
+
 	return hw->core;
 }