Fixed bug #116: "empty string with non-empty trailing context consumes code units".

Fixed bug #116: "empty string with non-empty trailing context consumes code units".

Prior to this commit backup of trailing context position was done
before advancing input position and re2c either had to emit
    YYCTXMARKER = YYCURSOR + 1;
(with default input API), or
    YYRESTORECTX ();
    YYSKIP ();
(with custom input API).

The problem is that sometimes initial state doesn't sdvance input position
at all. Now re2c emits context backup after advancing input position and it
no longer needs '+1' or 'YYSKIP' hacks. It always backups the correct position.