Fixes #414 - source map position fallback.

Fixes #414 - source map position fallback.

Apparently there is a problem with building source maps from input
source maps when the latter one does not have correct (?) selector
positions.

It happens to postcss which in case of such a source:

```css
selector-1,
selector-2,
selector-3
{
  color:red;
}
```

generates selector source position for the first selector only.

To fix this issue clean-css tries to find out correct selector
position going to lines above which may have a selector position.