[SHELL32] Initialize MenuItemInfoW struct in AddStaticContextMenusToMenu (#6737)

This prevents an issue where clicking on most of the icons on the desktop
that isn't the file explorer causes explorer to throw an exception,
and also prevents another issue where most applications won't launch from the start menu;
at least on MSVC builds running on VirtualBox 5.2.44.

^- I am pretty sure, that this was not always the case. The guilty rev is yet unknown though.

dll/win32/shell32/CDefaultContextMenu.cpp

@@ -493,13 +493,12 @@ CDefaultContextMenu::AddStaticContextMenusToMenu(
     UINT uFlags)
 {
     UINT ntver = RosGetProcessEffectiveVersion();
-    MENUITEMINFOW mii;
+    MENUITEMINFOW mii = { sizeof(mii) };
     UINT idResource;
     WCHAR wszVerb[40];
     UINT fState;
     UINT cIds = 0, indexFirst = *pIndexMenu;
 
-    mii.cbSize = sizeof(mii);
     mii.fMask = MIIM_ID | MIIM_TYPE | MIIM_STATE | MIIM_DATA;
     mii.fType = MFT_STRING;
     mii.dwTypeData = NULL;