Fix list out of bounds error in mail message parsing

Check list length before accessing the first two elements of
'dispositions'.
The list may have only a single element ('inline') or may be empty in
mailformed emails.

src/documents/mail.py

@@ -75,8 +75,9 @@ class Message(Loggable):
                 continue
 
             dispositions = content_disposition.strip().split(";")
-            if not dispositions[0].lower() == "attachment" and \
-               "filename" not in dispositions[1].lower():
+            if len(dispositions) < 2 or \
+               (not dispositions[0].lower() == "attachment" and
+               "filename" not in dispositions[1].lower()):
                 continue
 
             file_data = part.get_payload()