Fix for CTD due to division-by-zero

The value of iTheirStrength was computed and not checked for !=NULL before
being used for division. This applies the missing check in order to
prevent crashes here but this is more of an hotfix.

It should be checked why the context of this piece of code allowed 0 for
this variable and whether this could cause more problems.
Added a missing check for desired game logic

Units shall be able to doOpportunityFire() if they either can do air
combat or have an air range of greater than zero.
Fix for a CTD due to invalid city bombardment

Pointer-to-city (as the target for bombardement) was NULL and not checked
for before being dereferenced at one location. Added the necessary check.

This was probably an oversight from merging Super Forts into TTW.

Should check the source of Super Forts whether the same problem exists
there, too, or if the merge is incomplete in other places as well.
Fixed a CTD causing bug in AI city attack routine

This is a merge from K-Mod. The target city for an AI stack attack was
evaluated to NULL and later dereferenced. Added a check for nonNULL here.

Should check the corresponding K-Mod sources if we just imported this
problem or created it ourselves (maybe alongside others).
added TODO file