Check the mask flags in the item structure to determine how much
memory to read/write.

diff --git a/include/commctrl.h b/include/commctrl.h
index f5d29f2..e6ea707 100644
--- a/include/commctrl.h
+++ b/include/commctrl.h
@@ -2716,6 +2716,8 @@
 #define LVIF_PARAM              0x0004
 #define LVIF_STATE              0x0008
 #define LVIF_INDENT             0x0010
+#define LVIF_GROUPID            0x0100
+#define LVIF_COLUMNS            0x0200
 #define LVIF_NORECOMPUTE        0x0800
 #define LVIF_DI_SETITEM         0x1000