AI: Set number of cities in invalid requirement ranges to 0

The ranges adjacent and cadjacent aren't valid for buildings. Set the number of
cities in their range to 0 in stead of leaving them uninitialized.

See patch #4626

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@24726 a0f10bec-cc02-0410-94fc-a9cfff90b4cd