- creatures availability tests no longer check for built buildings.
Fixes #1650
- do not crash if town has 0 creatures on some dwelling level
- do not crash if dwelling for some level is not present in town at all


git-svn-id: http://svn.code.sf.net/p/vcmi/code/trunk/@3708 4dfc6b2a-8b31-0410-aa91-d6c70e3ac834