python: Fix unsafe utf-8 decodings

This prevents unsafe calls to decode for return value None in
get_authors/get_subject which would current throw an Exception. Original
patch modified by Sebastian Spaeth.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>

bindings/python/notmuch/tag.py

@@ -86,7 +86,7 @@ class Tags(object):
         if not nmlib.notmuch_tags_valid(self._tags):
             self._tags = None
             raise StopIteration
-        tag = Tags._get(self._tags).decode('utf-8')
+        tag = Tags._get(self._tags).decode('UTF-8')
         nmlib.notmuch_tags_move_to_next(self._tags)
         return tag
 

bindings/python/notmuch/thread.py

@@ -292,7 +292,10 @@ class Thread(object):
         """
         if self._thread is None:
             raise NotmuchError(STATUS.NOT_INITIALIZED)
-        return Thread._get_authors(self._thread).decode('UTF-8')
+        authors = Thread._get_authors(self._thread)
+        if authors is None:
+            return None
+        return authors.decode('UTF-8')
 
     def get_subject(self):
         """Returns the Subject of 'thread'
@@ -302,7 +305,10 @@ class Thread(object):
         """
         if self._thread is None:
             raise NotmuchError(STATUS.NOT_INITIALIZED)
-        return Thread._get_subject(self._thread).decode('UTF-8')
+        subject = Thread._get_subject(self._thread)
+        if subject is None:
+            return None
+        return subject.decode('UTF-8')
 
     def get_newest_date(self):
         """Returns time_t of the newest message date