Division by zero in TabDecorator CID 1210846

[kallisti5]

Per CID 1210846, there is a potential division by zero...

​http://cgit.haiku-os.org/haiku/tree/src/servers/app/decorator/TabDecorator.cpp#n431

In TabDecorator::_DistributeTabSize(float delta)

int32 nTabsWithMaxSize = 0;
.
.
for (int32 i = 0; i < fTabList.CountItems(); i++) {
  (make nTabsWithMaxSize larger)
}
.
.
minus /= nTabsWithMaxSize;

I don't have the time to study the code and fix it, but it was a big enough issue I decided it needed a ticket.