Don't rely on rule.position existing

[cw-peter]

In this code:

      rule.media = rule.media.replace(EXTENSION_RE, function(_, name) {
        var replacement = map[name];
        var column = rule.position.start.column;
        var line = rule.position.start.line;
        var source = rule.position.source;

you rely on position being defined, however some plugins mutate the AST and don't add the position property (@rework-pure-grids I'm looking at you...).

This breaks the plugin.

As the line and column are only used in an error message, it should be possible to check for their existence before using them.