[TYPO3-core] Fixed broken icons in Tools>User Admin

[Michael Stucki]

Hi Martin,

> Tools>User Admin didn't check if language icon was set for 'Limit to
> languages'. The error occured with the default language and any laguage
> without icon.
> 
> You cannot see a problem with Firefox but IE makes a hideos broken image
> box.
> 
> I fixed the bug by adding a check for the image path.

Fine. Additionally I have now wrapped the value check with strlen() to make
sure it is a non-empty string. Just in case...

- michael
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