Fix a rare crash on ReaderHighlight

The following crash is very rare and hard to reproduce. However, it can be avoided by checking `self.hold_pos` in `ReaderHighlight:onHold`.

```
lua config error: ./frontend/ui/reader/readerhighlight.lua:373: attempt to index upvalue 'pos' (a nil value)
stack traceback:
    ./frontend/ui/reader/readerhighlight.lua:373: in function 'inside_box'
    ./frontend/ui/reader/readerhighlight.lua:381: in function 'box_distance'
    ./frontend/ui/reader/readerhighlight.lua:393: in function 'getWordBoxIndices'
    ./frontend/ui/reader/readerhighlight.lua:427: in function 'getTextFromPositions'
    ./frontend/ui/reader/readerhighlight.lua:155: in function 'handleEvent'
    ./frontend/ui/widget/container.lua:43: in function 'propagateEvent'
    ./frontend/ui/widget/container.lua:55: in function 'handleEvent'
    ./frontend/ui/uimanager.lua:101: in function 'sendEvent'
    ./frontend/ui/uimanager.lua:249: in function 'run'
    ./reader.lua:251: in main chunk
```