Fix crash when changing node restrictions with DNS lookup in progress

Fixes bug 4259, bugfix on 0.2.2.25-alpha.  Bugfix by "Tey'".

Original message by submitter:

  Changing nodes restrictions using a controller while Tor is doing
  DNS resolution could makes Tor crashes (on WinXP at least). The
  problem can be repeated by trying to reach a non-existent domain
  using Tor:

    curl --socks4a 127.0.0.1:9050 inexistantdomain.ext

  .. and changing the ExitNodes parameter through the control port
  before Tor returns a DNS resolution error (of course, the following
  command won't work directly if the control port is password
  protected):

    echo SETCONF ExitNodes=TinyTurtle | nc -v 127.0.0.1 9051

  Using a non-existent domain is needed to repeat the issue so that
  Tor takes a few seconds for resolving the domain (which allows us to
  change the configuration). Tor will crash while processing the
  configuration change.

  The bug is located in the addressmap_clear_excluded_trackexithosts
  method which iterates over the entries of the addresses map in order
  to check whether the changes made to the configuration will impact
  those entries. When a DNS resolving is in progress, the new_adress
  field of the associated entry will be set to NULL. The method
  doesn't expect this field to be NULL, hence the crash.