Consider as corrupt a null-area bbox

The following crash happened with a pdf file containing some empty pages after I activated the "fit to width" zoom and scrolled near these empty pages. I don't know what exactly caused the regression, this file used to not cause any problems with previous revisions of koreader. I didn't do any bisect to see where it started. Discarding a bbox which has a null-area seems sensible, though. It prevents NaNs which currently appear in page state calculation which cause the page_states array not being populated and fixes the crash.
```
./luajit: frontend/apps/reader/modules/readerpaging.lua:620: attempt to index a nil value
stack traceback:
	frontend/apps/reader/modules/readerpaging.lua:620: in function 'onScrollPageRel'
	frontend/apps/reader/modules/readerpaging.lua:356: in function 'onPagingRel'
	frontend/apps/reader/modules/readerpaging.lua:174: in function 'handleEvent'
	frontend/ui/widget/container/widgetcontainer.lua:63: in function 'propagateEvent'
	frontend/ui/widget/container/widgetcontainer.lua:75: in function 'handleEvent'
	frontend/ui/uimanager.lua:121: in function 'sendEvent'
	frontend/ui/uimanager.lua:313: in function 'run'
	./reader.lua:201: in main chunk
	[C]: in function 'dofile'
	./koreader-base:22: in main chunk
	[C]: at 0x0000bbb8
```