sd-bus: check for potential integer overflow in KDBUS_ITEM_FOREACH()

For large values of item->size, the 'part' pointer can wrap around,
which results in an illegal pointer, but currently passes the for-loop
condition.

src/libsystemd/sd-bus/bus-kernel.h

@@ -31,7 +31,8 @@
 
 #define KDBUS_ITEM_FOREACH(part, head, first)                           \
         for (part = (head)->first;                                      \
-             (uint8_t *)(part) < (uint8_t *)(head) + (head)->size;      \
+             ((uint8_t *)(part) < (uint8_t *)(head) + (head)->size) &&  \
+                ((uint8_t *) part >= (uint8_t *) head);                 \
              part = KDBUS_ITEM_NEXT(part))
 
 #define KDBUS_ITEM_HEADER_SIZE offsetof(struct kdbus_item, data)