Corrected error message when parsing "var" inside "for".

Corrected error message when parsing "var" inside "for".

The "var_in" flag indicates parsing inside the "for" statement with expectation
of the "in" token, but it doesn't define the loop as a for-in one.  In fact,
that's unknown at the moment when this particular error message is emitted.

As a result, the following code:

  for (var eval = 10; eval; eval--);

produces the error message about for-in loop:

  SyntaxError: Identifier "eval" is forbidden in for-in var declaration

Actually, according to the specification "eval" isn't a keyword and shouldn't
cause any errors here.  But this bug is beyond the scope of the current fix.