Ticket #579 - Error messages encountered when using POSIX winsync

Bug description: posix_group_fix_memberuid_callback registered by
posixWinsyncCreateMemberOfTask calls an internal modify function
even if there are no attributes to fix up.  The attempt fails as
expected, but it logs cryptic errors in the error log:
  - slapi_modify_internal_set_pb: NULL parameter
  - allow_operation: component identity is NULL

Fix description: This patch skips calling the fix up internal
modify if there is no attributes to fix up.

https://fedorahosted.org/389/ticket/579

Reviewed by Rich (Thank you!!)

ldap/servers/plugins/posix-winsync/posix-group-task.c

@@ -249,6 +249,7 @@ posix_group_fix_memberuid_callback(Slapi_Entry *e, void *callback_data)
 
     char *dn = slapi_entry_get_dn(e);
     Slapi_DN *sdn = slapi_entry_get_sdn(e);
+    LDAPMod **mods = NULL;
 
 /* Clean out memberuids and dsonlymemberuids without a valid referant */
     rc = slapi_entry_attr_find(e, "memberuid", &muid_attr);
@@ -379,17 +380,19 @@ posix_group_fix_memberuid_callback(Slapi_Entry *e, void *callback_data)
         }
     }
 
-    Slapi_PBlock *mod_pb = slapi_pblock_new();
+    mods = slapi_mods_get_ldapmods_passout(smods);
+    if (mods) {
+        Slapi_PBlock *mod_pb = NULL;
+        mod_pb = slapi_pblock_new();
+        slapi_modify_internal_set_pb_ext(mod_pb, sdn, mods, 0, 0,
+                                        posix_winsync_get_plugin_identity(), 0);
 
-    slapi_modify_internal_set_pb_ext(mod_pb, sdn, slapi_mods_get_ldapmods_passout(smods), 0, 0,
-                                     posix_winsync_get_plugin_identity(), 0);
-
-    slapi_pblock_set(mod_pb, SLAPI_TXN, the_cb_data->txn);
-    slapi_modify_internal_pb(mod_pb);
-
-    slapi_pblock_get(mod_pb, SLAPI_PLUGIN_INTOP_RESULT, &rc);
-    slapi_pblock_destroy(mod_pb);
+        slapi_pblock_set(mod_pb, SLAPI_TXN, the_cb_data->txn);
+        slapi_modify_internal_pb(mod_pb);
 
+        slapi_pblock_get(mod_pb, SLAPI_PLUGIN_INTOP_RESULT, &rc);
+        slapi_pblock_destroy(mod_pb);
+    }
     slapi_mods_free(&smods);
 
     slapi_log_error(SLAPI_LOG_PLUGIN, POSIX_WINSYNC_PLUGIN_NAME,