Fixed a division by zero when player without any cities gains a tech.

See bug #21946


git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@24795 a0f10bec-cc02-0410-94fc-a9cfff90b4cd
Balanced civ2civ3 ruleset disasters.

Patch by David Fernandez <bardo>

See patch #4637


git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@24797 a0f10bec-cc02-0410-94fc-a9cfff90b4cd