Engine crash when building objects without mass

[Remus76]

The construction process of an object which has Mass=0 set in DefCore leads to an engine crash due to an unsecure division by this value in C4Object::Build. Same holds for leaving this value out of DefCore because 0 is the default then. I found such objects on CCAN so this could happen in real world scenarios.

Though I don't know the code style guidelines very well this can be fixed by using

const int32_t nonzeroMass = Def->Mass ? Def->Mass : 1;
DoCon(iLevel * iBuildSpeed * 150 / nonzeroMass, false, fNeedMaterial);

instead of https://github.com/legacyclonk/LegacyClonk/blob/master/src/C4Object.cpp#L1734 without breaking anything because

• this line only calculates the construction speed and the value is not used in any other code
• the object in this case would just behave like one with mass 1
• all other lines where Def->Mass is used are okay with 0

The fix is just a suggestion from my side. You can use any other value (or maybe something else?).