Fixes bug that was causing prefix to be added multiple times

[joaosamouco]

This bug was introduced by our PR #18. It was causing the namePrefix to be added multiple times to renamed variables in some scenarios (ie: IIFE).

Here is an example of what was happening:

(function () {
  var variableA = 1;
  var variableB = 2;
  var variableC = 3;
})();

When we mangled the tree with _ as namePrefix:

(function () {
  var _a = 1;
  var __b = 2;
  var ___c = 3;
})();

[michaelficarra]

What happens when your prefix is "a"?

[joaosamouco]

Just tested it and this is the output of the same example with a as prefix.

(function () {
  var aa = 1;
  var ab = 2;
  var ac = 3;
}());

[joaosamouco]

Took your comment in to consideration and moved the condition outside of the loop.

Tell me if you find something else that can be improved.