Fix non-swimmer zombies suddenly moving at half speed.

For the record, I have no idea WHY this fixes it. Or rather, I do know why this fixes it, but I don't know why the same fix can't be applied to swimming monsters on land.

But in map::combined_movecost, it does a /2 at the end of the calculation. This was missing from the formulae for the other two cases - monsters with MF_SWIMS and monsters that don't breathe. For whatever reason, when a regular enemy (say, a giant ant) was supposed to have a move cost of 100, the zombie would have 200.

Except that despite having twice the normal move cost, the swimmer zombie moves at normal speed.

I can't find where there's some other special exception for swimming monsters on land, so that probably means there's a better fix to this problem than this PR addresses, but for now this will serve as a duct-tape patch.