[freeciv-ai] another fix

["Per I. Mathisen" <per@xxxxxxxxxxx>]

This is another obviously correct fix for a possible bug that might make
trigger an assert in ai_unit_rampage(). It may be that an enemy city is
empty and within reach, but for some reason we are unable to enter it, if
bad ZOC or if get_defender() returns NULL while there are still non-allied
defenders present. Note that while the latter at the moment is a bug
(which does happen from time to time, don't know why), it will be normal
after the layers patch hits cvs.

  - Per

notallied2.diff
Description: Text document