* src/tgbatest/ltl2tgba.cc (main): Delete the minimized automaton.
* src/tgbaalgos/minimize.cc (minimize): Remove the call to
unregister_variable() at the end.  It was both
wrong (unregistering only the first variable) and useless ("delete
del_a" will unregister all these variables).  Use a map and a set
to keep track of free BDD variable and reuse them, otherwise the
algorithm would sometimes use more variables than allocated.