BUG/MINOR: resolvers: Use a null-terminated string to lookup in servers tree

BUG/MINOR: resolvers: Use a null-terminated string to lookup in servers tree

When we evaluate a DNS response item, it may be necessary to look for a
server with a hostname matching the item target into the named servers
tree. To do so, the item target is transformed to a lowercase string. It
must be a null-terminated string. Thus we must explicitly set the trailing
'\0' character.

For a specific resolution, the named servers tree contains all servers using
this resolution with a hostname loaded from a state file. Because of this
bug, same entry may be duplicated because we are unable to find the right
server, assigning this way the item to a free server slot.

This patch should fix the issue #1333. It must be backported as far as 2.2.