reference through null pointer and memory leak (related to ITS#7588)

Full_Name: John Hardin Version: master OS: Centos 6.4 URL: ftp://ftp.openldap.org/incoming/ Submission from: (NULL) (50.23.115.111)

The commit for ITS#7588 causes a crash if next_node is NULL:

/* Set the first entry to send for the next page */ so->so_tree = next_node; + next_node->avl_left = NULL;

next_node will be NULL if all entries have been sent, or if slapd_shutdown is true.

Another issue related to ITS#7588 is a memory leak when a sorted search with paged results is aborted. This may be because so->so_tree is not the root of the tree when free_sort_op() calls tavl_free().