Bug 613056 - fix coverify Defect Type: Null pointer dereferences issues 11892 - 11939

https://bugzilla.redhat.com/show_bug.cgi?id=613056
Resolves: bug 613056
Bug description: Fix coverify Defect Type: Null pointer dereferences issues 11892 - 11939
description: Catch possible NULL pointer in filter_optimize().

ldap/servers/slapd/filter.c

@@ -1522,7 +1522,7 @@ filter_optimize(Slapi_Filter *f)
 					if(f_child->f_choice != LDAP_FILTER_NOT)
 					{
 						/* we have a winner, do swap */
-						f_prev->f_next = f_child->f_next;
+						if (f_prev) f_prev->f_next = f_child->f_next;
 						f_child->f_next = f->f_list;
 						f->f_list = f_child;
 						break;