#24 wrong calculation of "negative" attack strengths on merge

[mranderson2007]

When merging two units with a "negativ" attack value (attack value is not suitable for attacking, but only for defending), the value is miscalculated. The bug is in unit.c, where it says:

unit->prop.atks[i] = (int)( weight1 * abs( unit->prop.atks[i] ) + weight2 * ( source->prop.atks[i] ) );

There is missing an abs() in the second addend.

[mranderson2007]

patch against trunk, fixing the bug