Callback isn't called when a file doesn't exist

If `path.join(base, opts.getSrc(el))` doesn't exist, `cb()` is never called.

```js
gulp.src(path.join(base, opts.getSrc(el)))
  .pipe(process)
  .pipe(replace(el, opts.template))
  .pipe(through.obj(function(file, enc, cb) {
    cb();
  }, done));
```

Example

```html
<link rel="stylesheet" href="mystyles.css">
<link rel="stylesheet" href="highlight.js/styles/default.css">
```

```js
gulp.src('path')
  .pipe(inline({ base: 'theme/', css: cssmin() }))
  .pipe(inline({ base: 'node_modules/', css: cssmin() }))
```

Fixed it by checking if it exists.