qjson: Apply nesting limit more sanely

The nesting limit from commit 29c75ddd "json-streamer: limit the
maximum recursion depth and maximum token count" applies separately to
braces and brackets.  This makes no sense.  Apply it to their sum,
because that's actually a measure of recursion depth.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Message-Id: <1448486613-17634-2-git-send-email-armbru@redhat.com>

qobject/json-streamer.c

@@ -64,8 +64,7 @@ static void json_message_process_token(JSONLexer *lexer, QString *token, JSONTok
          parser->bracket_count == 0)) {
         goto out_emit;
     } else if (parser->token_size > MAX_TOKEN_SIZE ||
-               parser->bracket_count > MAX_NESTING ||
-               parser->brace_count > MAX_NESTING) {
+               parser->bracket_count + parser->brace_count > MAX_NESTING) {
         /* Security consideration, we limit total memory allocated per object
          * and the maximum recursion depth that a message can force.
          */