Fix potentially undefined behavior

Really this time.

http://llvm.org/bugs/show_bug.cgi?id=8670

wayland/wayland-util.h

@@ -94,19 +94,20 @@ int wl_list_empty(struct wl_list *list);
 		 ((char *)&(sample)->member - (char *)(sample)))
 
 #define wl_list_for_each(pos, head, member)				\
-	for (pos = __container_of((head)->next, pos, member);		\
+	for (pos = 0, pos = __container_of((head)->next, pos, member);	\
 	     &pos->member != (head);					\
 	     pos = __container_of(pos->member.next, pos, member))
 
 #define wl_list_for_each_safe(pos, tmp, head, member)			\
-	for (pos = __container_of((head)->next, pos, member),		\
+	for (pos = 0, tmp = 0, 						\
+	     pos = __container_of((head)->next, pos, member),		\
 	     tmp = __container_of((pos)->member.next, tmp, member);	\
 	     &pos->member != (head);					\
 	     pos = tmp,							\
 	     tmp = __container_of(pos->member.next, tmp, member))
 
 #define wl_list_for_each_reverse(pos, head, member)			\
-	for (pos = __container_of((head)->prev, pos, member);		\
+	for (pos = 0, pos = __container_of((head)->prev, pos, member);	\
 	     &pos->member != (head);					\
 	     pos = __container_of(pos->member.prev, pos, member))