conversion: Add sanity check for page count

Add a sanity check at the end of the conversion from doc to pixels, to
ensure that the resulting document will have the same number of pages as
the original one.

Refs #560

dangerzone/conversion/doc_to_pixels.py

@@ -349,16 +349,26 @@ async def pdftoppm_progress_callback(line: bytes) -> None:
             timeout=timeout,
         )
 
-        self.update_progress("Converted document to pixels")
-
-        # Move converted files into /tmp/dangerzone
-        for filename in (
+        final_files = (
             glob.glob("/tmp/page-*.rgb")
             + glob.glob("/tmp/page-*.width")
             + glob.glob("/tmp/page-*.height")
-        ):
+        )
+
+        # XXX: Sanity check to avoid situations like #560.
+        if len(final_files) != 3 * num_pages:
+            raise errors.UnexpectedConversionError(
+                "The final number of pages does not correspond to the page count of the"
+                " original document"
+            )
+
+        # Move converted files into /tmp/dangerzone
+        for filename in final_files:
             shutil.move(filename, "/tmp/dangerzone")
 
+        self.update_progress("Converted document to pixels")
+
+
     async def install_libreoffice_ext(self, libreoffice_ext: str) -> None:
         self.update_progress(f"Installing LibreOffice extension '{libreoffice_ext}'")
         unzip_args = [