Fixed the error when Empty main runs with VerTracker (#494)

src/main/gov/nasa/jpf/listener/VarTracker.java

@@ -191,7 +191,9 @@ public void instructionExecuted(VM vm, ThreadInfo ti, Instruction nextInsn, Inst
       varId = ((JVMFieldInstruction)executedInsn).getFieldName();
 
       StackFrame frame = ti.getModifiableTopFrame();
-      frame.addOperandAttr(varId);
+      if (frame.getTopPos() >= 0) {
+        frame.addOperandAttr(varId);
+      }
 
 
     // here come the changes - note that we can't update the stats right away,

src/tests/gov/nasa/jpf/listener/VerTrackerTest.java

@@ -0,0 +1,21 @@
+package gov.nasa.jpf.listener;
+import gov.nasa.jpf.util.test.TestJPF;
+import org.junit.Test;
+
+class Emptymain{
+    public static void main(String[] args) {
+
+    }
+}
+public class VerTrackerTest extends TestJPF {
+    @Test
+      public void test_Emptymain(){
+        if(verifyNoPropertyViolation("+listener=gov.nasa.jpf.listener.VarTracker")){
+          Emptymain.main(null);
+        }
+    }
+    public static void main(String[] args) {
+        runTestsOfThisClass(null);
+    }
+
+}