dataman: allow writing empty items with nullptr pointer to data

src/modules/dataman/dataman.c

@@ -324,7 +324,9 @@ _write(dm_item_t item, unsigned char index, dm_persitence_t persistence, const v
 	buffer[1] = persistence;
 	buffer[2] = 0;
 	buffer[3] = 0;
-	memcpy(buffer + DM_SECTOR_HDR_SIZE, buf, count);
+	if (count > 0) {
+		memcpy(buffer + DM_SECTOR_HDR_SIZE, buf, count);
+	}
 	count += DM_SECTOR_HDR_SIZE;
 
 	len = -1;

src/modules/dataman/dataman.h

@@ -133,7 +133,7 @@ extern "C" {
 	   in this header, but till I find it a better home here it is */
 
 	typedef struct {
-		char current_offboard_waypoint_id;	/* the index of the active offboard waypoint data */
+		char offboard_waypoint_id;	/* the index of the active offboard waypoint data */
 							/* (DM_KEY_WAYPOINTS_OFFBOARD_n) or -1 for none */
 	} persistent_system_state_t;