For issue #485.
* spot/tl/formula.cc, spot/tl/formula.hh: Catch min/max overflow
when the operators are constructed. Also disable travial
simplification rules that would create such overflow.
For instance x[*200][*2] will not become x[*400] anymore.
* python/spot/impl.i: Catch std::overflow_error.
* tests/core/equals.test, tests/python/except.py: Add test cases.