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

On case 'fdt memory' is invoked without parameters, argv[2]/argv[3]
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