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

frontend/ui/widget/dictquicklookup.lua

@@ -75,7 +75,7 @@ function DictQuickLookup:canSearch()
         end
     else
         -- This is to prevent an ineffective button when we're launched from the Reader's menu.
-        if self.ui.highlight.selected_text then
+        if self.highlight then
             return true
         end
     end

frontend/ui/widget/textwidget.lua

@@ -214,6 +214,9 @@ function TextWidget:_measureWithXText()
                 -- no bold: xtext does synthetized bold with normal metrics
         end
         local max_width = self.max_width - reserved_width
+        if max_width <= 0 then -- avoid _xtext:makeLine() crash
+            max_width = self.max_width
+        end
         if self.truncate_left then
             line_start = self._xtext:getSegmentFromEnd(max_width)
         end
@@ -311,7 +314,6 @@ end
 function TextWidget:setText(text)
     if text ~= self.text then
         self.text = text
-        self._updated = false
         self:free()
     end
 end
@@ -324,7 +326,6 @@ dbg:guard(TextWidget, "setText",
 function TextWidget:setMaxWidth(max_width)
     if max_width ~= self.max_width then
         self.max_width = max_width
-        self._updated = false
         self:free()
     end
 end
@@ -378,6 +379,7 @@ function TextWidget:free()
         self._xtext:free()
         self._xtext = nil
     end
+    self._updated = false
 end
 
 function TextWidget:onCloseWidget()