External SAT back-end: clear clauses after writing DIMACS

There is no need to hold on to clauses. Freeing up memory makes it
available to the external SAT process instead.

Author: tautschnig

src/solvers/sat/external_sat.cpp

@@ -177,6 +177,7 @@ external_satt::resultt external_satt::do_prop_solve()
   // create a temporary file
   temporary_filet cnf_file("external-sat", ".cnf");
   write_cnf_file(cnf_file());
+  clauses.clear();
   auto output = execute_solver(cnf_file());
   return parse_result(output);
 }