Avoid crash if trying to open a non-existent file

which currently might occur by clicking in the history entry of a file which was removed
Merge pull request #458 from thotypous/master

Avoid crash if trying to open a non-existent file
Merge pull request #459 from chrox/master

add text highlight for credocument
Merge pull request #460 from chrox/master

fix nil page state when saving settings in ReaderPaging
Merge pull request #461 from chrox/master

update koreader-base
Avoid PageUpdate from receiving out-of-range page

Otherwise, current_page can acquire negative values, or values greater
than the page count, when the user tries to go back the first page
or past the last page.
Restore north and south swipe gestures

Might be useful in pages covered with links, which make it difficult
to tap in the correct point for flipping the page.
Merge pull request #462 from thotypous/master

Fix out-of-range page and tweak swipe
page links for crereader and pdfreader
Swipe right will return to previous page or position.