parser: add necessary checks to operator() to stop accepting nonsense.

parser: add necessary checks to operator() to stop accepting nonsense.

Since the parser is recursive parse_* methods (in particular,
parse_binop_rhs()) does NOT check if the last unparsed token is valid.
Thus parse_expression() stops if it encounters an unknown token. That's
why parser::operator() needs to make sure nothing is left in the input
stream.