Fix a few possible crashes

- TextWidget: avoid crash with small max_width (could happen
  when opening menu on a small emulator window)
- TextWidget: avoid crash if re-used after :free()
- ReaderHighlight:clear(): fix possible crash when
  scheduled and run after document closed
- DictQuickLookup: minor canSearch() tweak ('selected_text'
  is normally available only on multiple words selection,
  but is currently available on single word selection thanks
  to some unrelated side effect)

frontend/apps/reader/modules/readerhighlight.lua

@@ -379,6 +379,10 @@ function ReaderHighlight:clear(clear_id)
         end
     end
     self.clear_id = nil -- invalidate id
+    if not self.ui.document then
+        -- might happen if scheduled and run after document is closed
+        return
+    end
     if self.ui.document.info.has_pages then
         self.view.highlight.temp = {}
     else