Fix an off by one error on negative indexs to __getitem__
http://code.google.com/p/ipaddr-py/issues/detail?id=15
diff --git a/Lib/ipaddr.py b/Lib/ipaddr.py
index b5518ed..7289e45 100644
--- a/Lib/ipaddr.py
+++ b/Lib/ipaddr.py
@@ -209,6 +209,7 @@
                 raise IndexError
             return self._string_from_ip_int(self.network + n)
         else:
+            n += 1
             if self.broadcast + n < self.network:
                 raise IndexError
             return self._string_from_ip_int(self.broadcast + n)