Patch to fix rare desync.

Hi,

this desync happens when bots make a grid for resource gathering on a
map boundary (y=0).
It can be reproduced by running a game with two glob2 clients through
YOG:
 - map "6P Balanced";
 - 4 Nicowars.
(Create a game with one instance and join your game with other instance
of glob2.)
It'll crash with desync in 5-10 min almost always. If it's still running
for more than 20 min - try again.

Desync happens because in src/AINicowar.cpp NewNicowar::update_farming()
water gradient's get_height() (see src/AIEcho.h Gradient::get_pos()) is
called with negative coordinates. And it can't handle it correctly.

beta5 and tip are affected.
beta4's gradient doesn't handle negative coords too, but I don't see it
being called like that. beta4 doesn't crash on this map.

patch http://pastebin.com/2BGv3Mbi