Improve SMTInterpol prover cleanup when closing

Author: BaierD

src/org/sosy_lab/java_smt/solvers/smtinterpol/SmtInterpolAbstractProver.java

@@ -217,7 +217,8 @@ public ImmutableMap<String, String> getStatistics() {
   public void close() {
     if (!closed) {
       annotatedTerms.clear();
-      env.pop(size());
+      env.resetAssertions();
+      env.exit();
     }
     super.close();
   }