BUG: IndexError for getPage() of decryped file (#359)

ISSUE: Problem appears because _flatten() method sets self.flattenedPages before it tries to get pages and doesn't set it back to None in case of error. This PR just makes _flatten() to set self.flattenedPages to an empty array after it successfully got pages.

FIX: Call `self.flattenedPages` after calling `catalog["/Pages"].getObject()`

Closes #327

Author: denis-osipov

PyPDF2/pdf.py

@@ -1575,9 +1575,11 @@ def _flatten(self, pages=None, inherit=None, indirectRef=None):
         if inherit is None:
             inherit = dict()
         if pages is None:
-            self.flattenedPages = []
+            # Fix issue 327: set flattenedPages attribute only for
+            # decrypted file
             catalog = self.trailer["/Root"].getObject()
             pages = catalog["/Pages"].getObject()
+            self.flattenedPages = []
 
         t = "/Pages"
         if "/Type" in pages: