Fix a memory leak on failure to WDBA-minimize.

* src/tgbaalgos/minimize.cc (minimize_obligation): Delete the powerset
automaton when we return 0 because we know if the result is correct
and don't have the formulae to check it.  Reported by Étienne Renault.

src/tgbaalgos/minimize.cc

@@ -638,6 +638,7 @@ namespace spot
     if (!f && !aut_neg_f)
       {
 	// We do not now if the minimization is safe.
+	delete min_aut_f;
 	return 0;
       }