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).

frontend/document/koptinterface.lua

@@ -180,6 +180,11 @@ function KoptInterface:getSemiAutoBBox(doc, pageno)
         auto_bbox.x1 = auto_bbox.x1 + bbox.x0
         auto_bbox.y1 = auto_bbox.y1 + bbox.y0
         logger.dbg("Semi-auto detected bbox", auto_bbox)
+        local native_size = Document.getNativePageDimensions(doc, pageno)
+        if (auto_bbox.x1 - auto_bbox.x0)/native_size.w < 0.1 and (auto_bbox.y1 - auto_bbox.y0)/native_size.h < 0.1 then
+            logger.dbg("Semi-auto detected bbox too small, using manual bbox")
+            auto_bbox = bbox
+        end
         page:close()
         Cache:insert(hash, CacheItem:new{ semiautobbox = auto_bbox })
         kc:free()