fixes bug 313963 "last-update.log not created if it does not already exist" r=bsmedberg

toolkit/mozapps/update/src/nsUpdateService.js.in

@@ -349,6 +349,7 @@ function cleanUpUpdatesDir() {
         var dir = f.parent.parent;
         var logFile = dir.clone();
         logFile.append(FILE_LAST_LOG);
+        if (logFile.exists())
           logFile.remove(false);
         f.copyTo(dir, FILE_LAST_LOG);
       }