check_message: Reject null bytes in message content.

Postgres doesn't like them, we don't have an obvious way to escape
them, and they tend to be sent by buggy tools where it'd be better for
the user to get an error.

This fixes a 500 we were getting occasionally.

zerver/lib/actions.py

@@ -1553,6 +1553,9 @@ def check_message(sender, client, addressee,
     message_content = message_content_raw.rstrip()
     if len(message_content) == 0:
         raise JsonableError(_("Message must not be empty"))
+    if '\x00' in message_content:
+        raise JsonableError(_("Message must not contain null bytes"))
+
     message_content = truncate_body(message_content)
 
     if realm is None:
@@ -1652,6 +1655,7 @@ def _internal_prep_message(realm, sender, addressee, content):
     messages together as one database query.
     Call do_send_messages with a list of the return values of this method.
     """
+    # Remove any null bytes from the content
     if len(content) > MAX_MESSAGE_LENGTH:
         content = content[0:3900] + "\n\n[message was too long and has been truncated]"