Building SubnetBuilder after Optimization failed

related MR !299 (closed). To run:

./build/test/utest --gtest_filter=GraphMlSubnetParserTest.*

Message:

utest: /home/alexey/Work/utopia-eda/src/gate/model2/subnet.cpp:557: size_t eda::gate::model::SubnetBuilder::allocEntry(eda::gate::model::CellTypeID, const LinkList&): Assertion `!cell.isOut()' failed.
Aborted (core dumped)