x86: fix missing led variable warning during boot

Fixes following warning during the boot:

 WARNING: Variable 'led' does not exist or is not an array/object

Signed-off-by: Petr Štetiar <ynezz@true.cz>

target/linux/x86/base-files/etc/diag.sh

@@ -20,6 +20,8 @@ preinit_match_diag_led() {
 	json_init
 	json_load "$(cat $CFG)"
 	json_get_keys keys led
+	json_is_a led object || return
+
 	json_select led
 	for key in $keys; do
 		json_select "$key"