cmd: spi: check return value of strdup

Check return value of strdup.

Signed-off-by: Peng Fan <van.freenix@gmail.com>
Cc: Przemyslaw Marczak <p.marczak@samsung.com>
Cc: Bin Meng <bmeng.cn@gmail.com>
Cc: Heiko Schocher <hs@denx.de>
Cc: Simon Glass <sjg@chromium.org>
Cc: Stefan Roese <sr@denx.de>
Cc: Tom Rini <trini@konsulko.com>