Prevent PDF from overwriting num_pages in Qubes

This should only affect the alpha version of Qubes OS (in containers
it only allows the attacker to control the timeout). In short, an
attacker could have PDF metadata that would show before "Pages:" in
the `pdfinfo` command output and this would essentially override the
number of pages measured in the server. This could enable the attacker
to shorten the number of pages of a document for example.

Fixes #565

dangerzone/conversion/doc_to_pixels.py

@@ -254,7 +254,7 @@ class DocumentToPixels(DangerzoneConverter):
             timeout=timeout,
         )
 
-        search = re.search(r"Pages:\s*(\d+)\s*\n", stdout.decode())
+        search = re.search(r"^Pages:\s*(\d+)\s*\n", stdout.decode(), re.MULTILINE)
         if search is not None:
             num_pages: int = int(search.group(1))
         else: