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>
diff --git a/cmd/fdt.c b/cmd/fdt.c
index 734c9b3..f257bee 100644
--- a/cmd/fdt.c
+++ b/cmd/fdt.c
@@ -611,6 +611,10 @@
 	} else if (strncmp(argv[1], "me", 2) == 0) {
 		uint64_t addr, size;
 		int err;
+
+		if (argc != 4)
+			return CMD_RET_USAGE;
+
 		addr = simple_strtoull(argv[2], NULL, 16);
 		size = simple_strtoull(argv[3], NULL, 16);
 		err = fdt_fixup_memory(working_fdt, addr, size);