Icon existence check in lte-ie7.js

[peter....@gmail.com]

This change to lte-ie7.js adds a check, if the icon really exists in the icons array (or we get some strange empty span tags).

- if (c) {
+ if (c && icons[c[0]] != null) {

This change request is also referenced at https://github.com/joomla/joomla-cms/pull/538/files.