[loose parser] Fix interpretation of `f."`

Before this the ast produced by parse_dammit crashed in the following
code, as Uglify correctly noticed that f."" is invalid.

    sample = 'f."';

    loose = require('acorn/acorn_loose');
    uglify = require('uglify-js');

    out = new uglify.OutputStream();
    ast = loose.parse_dammit(sample);
    ast = uglify.AST_Node.from_mozilla_ast(ast);
    ast.print(out);
    // TypeError: Cannot call method 'toString' of undefined
    // member_exp.computed = false && member_exp.property == ""

    console.log(out.toString());

After this the round-tripped AST looks like: `t.✖;"";`, which is
consistent with how `foo.{` is parsed.

I also considered making it parse as t[""], but as this only turns up in
the wild when people try to use multiline strings, I felt it was better
to be obviously wrong.