vacuum: in ctdb_vacuum_db, fix the length of the array of vacuum fetch lists

vacuum: in ctdb_vacuum_db, fix the length of the array of vacuum fetch lists

This patch fixes segfaults in the vacuum child when at least one
node has been stopped or removed from the cluster:

The size of the vnn_map is only the number of active nodes
(that can be lmaster). But the node numbers that are referenced
by the vnn_map spread over all configured nodes.

Since the array of vacuum fetch lists is referenced by the
key's lmaster's node number later on, the array needs to
be of size num_nodes instad of vnn_map->size.