cmd: fdt: Check argc before accessing argv in fdt bootcpu

On case 'fdt bootcpu' is invoked without parameters, argv[2] is not
valid and this command would SEGFAULT in sandbox environment. Add
missing argc test to avoid the crash and rather print usage help
message.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
1 file changed