Fix vboot compilation with coreboot-sdk

BUG=none
BRANCH=none
TEST=Upstream builder does not fail with uninitialized variable anymore like this:
 firmware/lib/vboot_ui_menu.c: In function 'vb2_get_current_menu_size':
 firmware/lib/vboot_ui_menu.c:255:15: error: 'temp_menu' may be used uninitialized in this function [-Werror=maybe-uninitialized]
    *menu_array = temp_menu;
    ~~~~~~~~~~~~^~~~~~~~~~~
 cc1: all warnings being treated as errors

Signed-off-by: Stefan Reinauer <reinauer@chromium.org>

Change-Id: Idb1690c7b0becf3556cad379160fe152707ae843
Reviewed-on: https://chromium-review.googlesource.com/777805
Commit-Ready: Stefan Reinauer <reinauer@google.com>
Tested-by: Stefan Reinauer <reinauer@google.com>
Reviewed-by: Martin Roth <martinroth@chromium.org>
Reviewed-by: Shelley Chen <shchen@chromium.org>
diff --git a/firmware/lib/vboot_ui_menu.c b/firmware/lib/vboot_ui_menu.c
index 11254cd..851e905 100644
--- a/firmware/lib/vboot_ui_menu.c
+++ b/firmware/lib/vboot_ui_menu.c
@@ -221,7 +221,7 @@
 void vb2_get_current_menu_size(VB_MENU menu, char ***menu_array,
 				    uint32_t *size)
 {
-	char **temp_menu;
+	char **temp_menu = NULL;
 
 	switch(menu) {
 	case VB_MENU_DEV_WARNING: