Editorial: use proper comparison operation in LabelledEvaluation of LabelledStatement

[hyp3rflow]

This PR adds missing ~empty~ guard in LabelledEvaluation for LabelledStatement.

Currently, LabelledEvaluation for LabelledStatement is defined as follows:

1. Let label be the StringValue of LabelIdentifier.
2. Let newLabelSet be the list-concatenation of labelSet and « label ».
3. Let stmtResult be Completion(LabelledEvaluation of LabelledItem with argument newLabelSet).
4. If stmtResult.[[Type]] is break and SameValue(stmtResult.[[Target]], label) is true, then
	a. Set stmtResult to NormalCompletion(stmtResult.[[Value]]).
5. Return ? stmtResult.

According to the definition, SameValue, used in step 4, requires two arguments which are both ECMAScript langauge value. In step 3, stmtResult.[[Target]] can be ~empty~ if there is no LabelIdentifier in BreakStatement. Consequently, providing stmtResult.[[Target]] as the argument for SameValue should not be allowed.

Following ECMAScript snippet can occur this case:

switch (1) {
  case 1:
    foo: { break; } // stmtResult.[[Type]] is break and stmtResult.[[Target]] is ~empty~
}

So fix this by adding more condition in step 4 for guard.

[michaelficarra]

We should be able to just change the SameValue comparison to an "is" comparison. See https://tc39.es/ecma262/#sec-identity.

[hyp3rflow]

@michaelficarra I updated as you commented. :)