Web Inspector: Exception when turning on pretty print without any displayed source in the editor
https://bugs.webkit.org/show_bug.cgi?id=113136

Patch by PhistucK <phistuck@chromium.org> on 2013-03-25
Reviewed by Vsevolod Vlasov.

No new tests.
Added a check for the existence of currently edited document.

* inspector/ScriptsPanel.js:
(WebInspector.ScriptsPanel.prototype._toggleFormatSource):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@146749 268f45cc-cd09-0410-ab3c-d52691b4dbfc