Fixed the error when Empty main runs with VerTracker

[shreyasCs012]

Description :

This PR fixes the error when we run an empty main set to a VerTracker listener.

Issue Fixed :

#380

Testing :

I added a test that calls the empty main method setting the listener to VerTracker.

###Fix :

• The error was reflected because the addOperandAttr() method was called in the VerTracker without checking if the
  operand stack was empty.

• The if condition (frame.getTopPos() >= 0) checks for the above undetected condition.

[cyrille-artho]

Looks good! I think we need the same check in both places where addOperandAttr is called; see my comment.
We unfortunately don't have a test to ascertain this, though.

[shreyasCs012]

I think Before the call for addOperandAttr(varId); there is a call for the peek() method which from the look of it returns the element in the slots array of index top

 public int peek () {
    return slots[top];
  }

if the top value does not change before the call for the addOperandAttr(varId); method then either an error should hit in that call for the peek() method or if the error does not hit there should be no problem when the call for addOperandAttr(varId);.

Again I am assuming that the top variable is not being changed in between the call for the peek() method and the addOperendAttr()

So, I think a check in the call for peek() would be more suitable than a check in the call for addOperandAttr(varId)

[cyrille-artho]

Good observation. I noticed now also that the instruction must be ALOAD, which means that there is a local variable slot available (because the bytecode verifier would have rejected the code earlier, when it was verified after loading, if that were not the case).
We can therefore merge this PR as it is. Thank you very much for your work!