Fix check_victory (for mattsc)

*** Please don't merge this, this is in a testing and review stage. ***

This branch is intended to address concerns raised by mattsc on irc. See http://www.wesnoth.org/irclogs/2014/03/%23wesnoth-dev.2014-03-26.log
and the day before for discussion.

The actual issue which is addressed -- sides shouldn't be defeated if they have no_leader = yes and still have units. mattsc has several micro ai scenarios which use ai sides with no leaders interacting. More generally it is legitimately a logic error and a bug to end such scenarios prematurely.

However, according to grep "victory_when_enemies_defeated=no" appears in the following campaigns
DiD
EI
NR
UtBS
tutorial
Two Brothers
Liberty
TSG
TRoW
LoW
Delfador's Memoirs
THoT
Dead Water
SoF
HttT

Therefore some testing is in order.