avoid possible exception if selection is undefined

defensive programming only

services/web/public/coffee/ide/pdfng/directives/pdfViewer.coffee

@@ -441,11 +441,11 @@ define [
 						return true
 
 				_hasSelection = () ->
-					selection = window.getSelection()
+					selection = window.getSelection?()
 					# check the selection type in preference to using
 					# selection.toString() as the latter is "" when the
 					# selection is hidden (e.g. while viewing logs)
-					return _isSelectionWithinPDF(selection) and selection.type is 'Range'
+					return selection? and _isSelectionWithinPDF(selection) and selection.type is 'Range'
 
 				_isSelectionWithinPDF = (selection) ->
 					if selection.rangeCount == 0