Prohibitting access to empty pointer in recursive image edge construction

src/ITAPropagationPathSim/CombinatedModel/ImageConstructor.cpp

@@ -298,6 +298,9 @@ namespace ITAPropagationPathSim
 
 			bool RecursiveConstructImageEdges(CPropagationShapeShared& pPropagationShape)
 			{
+				if( pPropagationShape == nullptr )
+					return false;
+
 				//if the type is face, just go one level deeper
 				if (pPropagationShape->iShapeType == CPropagationShape::EDGE)
 				{