Fix crash with kopt semi-auto crop on blank page
Fix crash with kopt semi-auto crop on blank page

When such blank page has only a few black pixels (dust), it would result in
a really small cropped area, than when scaled to fit screen, would cause a
crash with :
    ./ffi/mupdf.lua:63: could not allocate pixmap:
         malloc of array (451342 x 612558 bytes) failed (integer overflow) (1)
Fix similar to what is done in KoptInterface:getAutoBBox(): when cropped area too small,
fall back to whole page (or here: to user's manually set area).

mihailim/koreader,Markismus/koreader,robert00s/koreader,NiLuJe/koreader,houqp/koreader,Frenzie/koreader,lgeek/koreader,NiLuJe/koreader,koreader/koreader,Hzj-jie/koreader,pazos/koreader,Frenzie/koreader,mwoz123/koreader,koreader/koreader,poire-z/koreader,poire-z/koreader,apletnev/koreader