Town Network: Fix issue with DEBUG defined and empty town_array entries

Fix for #21, thanks to sraboy for finding this error.

Now using TownArray::is_deleted to check for null entries; this is in line with the rest of the code.

I double checked, because is_deleted also checks if population > 0, but any town whose population reaches 0 is immediately deinitialised, which means it is removed from the town_network, so this check fixes 'a double bug': 1) with NULL entries with DEBUG defined, and 2) with dying towns that were saved to file.