cmd: check argc for acpi dump

'acpi dump' without parameter results in a NULL dereference. Check the
number of arguments.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/cmd/acpi.c b/cmd/acpi.c
index 8997041..0c14409 100644
--- a/cmd/acpi.c
+++ b/cmd/acpi.c
@@ -160,6 +160,9 @@
 	char sig[ACPI_NAME_LEN];
 	int ret;
 
+	if (argc < 2)
+		return CMD_RET_USAGE;
+
 	name = argv[1];
 	if (strlen(name) != ACPI_NAME_LEN) {
 		printf("Table name '%s' must be four characters\n", name);