credocument: deal with loadDocument() failures (#3570)

cre:loadDocument() may fail in recognizing the document format, and
koreader would previously keep calling other methods on it, which would
make crengine segfaults. We now check loadDocument success at the
various places it is called, and try to deal the best way we can when it fails.

frontend/document/credocument.lua

@@ -109,6 +109,11 @@ function CreDocument:init()
     -- set fallback font face
     self._document:setStringProperty("crengine.font.fallback.face", self.fallback_font)
 
+    -- We would have liked to call self._document:loadDocument(self.file)
+    -- here, to detect early if file is a supported document, but we
+    -- need to delay it till after some crengine settings are set for a
+    -- consistent behaviour.
+
     self.is_open = true
     self.info.has_pages = false
     self:_readMetadata()
@@ -117,9 +122,11 @@ end
 
 function CreDocument:loadDocument()
     if not self._loaded then
-        self._document:loadDocument(self.file)
-        self._loaded = true
+        if self._document:loadDocument(self.file) then
+            self._loaded = true
+        end
     end
+    return self._loaded
 end
 
 function CreDocument:render()
@@ -160,7 +167,9 @@ end
 
 function CreDocument:getCoverPageImage()
     -- don't need to render document in order to get cover image
-    self:loadDocument()
+    if not self:loadDocument() then
+        return nil -- not recognized by crengine
+    end
     local data, size = self._document:getCoverPageImageData()
     if data and size then
         local Mupdf = require("ffi/mupdf")