list: make xorg_list cope with zero-initialized list heads

Most xorg_list operations already tolerate a list head that was merely
zero-initialized (next == prev == NULL) instead of run through
xorg_list_init(): xorg_list_add()/xorg_list_append() auto-init via
__xorg_list_autoinit(), and xorg_list_for_each_entry(), xorg_list_is_empty()
and xorg_list_present() guard against a NULL head->next.

Two paths were still missed, so a head embedded in a calloc()'d struct could
crash before anything was ever added to it:

  - xorg_list_for_each_entry_safe() guarded its loop *condition* with
    (head)->next != NULL, but its init clause unconditionally computed
    tmp = container_of(pos->member.next, ...). On a zeroed head, pos resolves
    to container_of(NULL) and reading pos->member.next dereferences address 0,
    crashing before the guard is evaluated. Seed tmp from
    (head)->next ? pos->member.next : NULL so an empty/zeroed list iterates
    zero times without dereferencing anything.

  - xorg_list_del() dereferenced entry->prev/entry->next directly, so resetting
    a zeroed head (the documented "reset to empty list" use) would NULL-deref.
    Auto-init it first, consistent with xorg_list_add()/xorg_list_append().

With these, a zero-initialized xorg_list head behaves as a valid empty list
across the whole API, so an embedded list head no longer strictly requires an
explicit xorg_list_init() call to be safe.

Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>

include/list.h

@@ -215,6 +215,7 @@ __xorg_list_del(struct xorg_list *prev, struct xorg_list *next)
 static inline void
 xorg_list_del(struct xorg_list *entry)
 {
+    __xorg_list_autoinit(entry);
     __xorg_list_del(entry->prev, entry->next);
     xorg_list_init(entry);
 }
@@ -379,7 +380,7 @@ xorg_list_append_ndup(struct xorg_list *entry, struct xorg_list *head)
 #define xorg_list_for_each_entry_safe(pos, tmp, head, member)		\
     for ((pos) = NULL,                                                    \
          (pos) = __container_of((head)->next, (pos), member),		\
-	 (tmp) = __container_of((pos)->member.next, (pos), member);		\
+	 (tmp) = __container_of((head)->next ? (pos)->member.next : NULL, (pos), member); \
 	 (((head)->next != NULL) && (&(pos)->member != (head)));		\
 	 (pos) = (tmp), (tmp) = __container_of((pos)->member.next, (tmp), member))