misc: fastrpc: check return value of devm_kasprintf()

misc: fastrpc: check return value of devm_kasprintf()

devm_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: 3abe3ab3cdab ("misc: fastrpc: add secure domain support")
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Link: https://lore.kernel.org/r/20230615102546.581899-1-claudiu.beznea@microchip.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>