Abort when excuting a code like "not 5<6"

Created by: MolmcChen

When I type a code like "if not 5<6 then", the aseba studio will direct abort. Logically "not 5<6" will return a bool value of FALSE, although it is meaningless. The bug derived from the function "Node* UnaryArithmeticNode::optimize(std::wostream* dump)" in the file tree-optimized.cpp. The function set this case into the "default" part, i.e., once the optimized children[0] "5<6" return a node of ImmediateNode type, the studio will abort().