clk: vc5: check memory returned by kasprintf()

clk: vc5: check memory returned by kasprintf()

kasprintf() returns a pointer to dynamically allocated memory.
Pointer could be NULL in case allocation fails. Check pointer validity.
Identified with coccinelle (kmerr.cocci script).

Fixes: f491276a5168 ("clk: vc5: Allow Versaclock driver to support multiple instances")
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Link: https://lore.kernel.org/r/20230530093913.1656095-2-claudiu.beznea@microchip.com
Reviewed-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>