Added protection against null pointer dereferencing.
Modified BRepOffset_MakeOffset::MakeThickSolid method.
Fixed the problem with negative volume values.
Fixed several unit tests and added a new one.
Multiple changes in BRepOffset_MakeOffset algorithm:
- correct fusing vertices;
- correct building intersection edges;
- correct intersecting new edges in 2D;
- correct building wire on a new face;
- correct building history.