Fix rare bugs identified by fuzzing

I have "manually fuzzed" the reader with input events after opening a number of files in different formats (an automatic fuzzer would be a nice tool to write in the future :P).

4 different errors were produced. All of them are very hard to reproduce. This PR fixes 3 of them.

The following I am not sure what would be the best way to prevent, because the "PageUpdate" event would be received in different objects and might produce other effects:

```
./luajit: ./frontend/document/document.lua:90: cannot open page #30, out of range (1-11)
stack traceback:
	[C]: in function 'openPage'
	./frontend/document/document.lua:90: in function 'getNativePageDimensions'
	./frontend/document/document.lua:109: in function 'getPageArea'
	./frontend/ui/reader/readerview.lua:426: in function 'recalculate'
	./frontend/ui/reader/readerview.lua:574: 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/reader/readerlink.lua:85: 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:119: in function 'sendEvent'
	./frontend/ui/uimanager.lua:307: in function 'run'
	./reader.lua:196: in main chunk
	[C]: in function 'dofile'
	./koreader-base:37: in main chunk
	[C]: at 0x0000c604
```

The following 3 errors are hopefully avoided by the changes in this PR. Tracking the root cause of the errors would be better, but as we are next to release and the bugs are hard to reproduce, let's fix them this way by now.

The first error below is the most common. I have actually ran into it even before the "fuzzing", but I was never able to reproduce it consistently.

```
./luajit: ./frontend/ui/reader/readerfooter.lua:80: attempt to perform arithmetic on field 'pageno' (a string value)
stack traceback:
	./frontend/ui/reader/readerfooter.lua:80: in function 'updateFooter'
	./frontend/ui/reader/readerfooter.lua:91: 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/widget/container/widgetcontainer.lua:63: in function 'propagateEvent'
	./frontend/ui/widget/container/widgetcontainer.lua:75: in function 'handleEvent'
	./frontend/ui/reader/readerlink.lua:85: 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:119: in function 'sendEvent'
	./frontend/ui/uimanager.lua:307: in function 'run'
	./reader.lua:196: in main chunk
	[C]: in function 'dofile'
	./koreader-base:37: in main chunk
	[C]: at 0x0000c604
```

```
./luajit: ./frontend/ui/reader/readerlink.lua:56: attempt to index local 'pos' (a nil value)
stack traceback:
	./frontend/ui/reader/readerlink.lua:56: 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:119: in function 'sendEvent'
	./frontend/ui/uimanager.lua:307: in function 'run'
	./reader.lua:196: in main chunk
	[C]: in function 'dofile'
	./koreader-base:37: in main chunk
	[C]: at 0x0000c604
```

```
./luajit: ./frontend/ui/reader/readerdictionary.lua:41: attempt to index local 'box' (a nil value)
stack traceback:
	./frontend/ui/reader/readerdictionary.lua:41: in function 'showDict'
	./frontend/ui/reader/readerdictionary.lua:31: in function 'stardictLookup'
	./frontend/ui/reader/readerdictionary.lua:13: 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/reader/readerhighlight.lua:297: in function 'lookup'
	./frontend/ui/reader/readerhighlight.lua:320: 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:119: in function 'sendEvent'
	./frontend/ui/uimanager.lua:307: in function 'run'
	./reader.lua:196: in main chunk
	[C]: in function 'dofile'
	./koreader-base:37: in main chunk
	[C]: at 0x0000c604
```