qjson: Don't crash when input exceeds nesting limit

We limit nesting depth and input size to defend against input
triggering excessive heap or stack memory use (commit 29c75dd
json-streamer: limit the maximum recursion depth and maximum token
count).  However, when the nesting limit is exceeded,
parser_context_peek_token()'s assertion fails.

Broken in commit 65c0f1e "json-parser: don't replicate tokens at each
level of recursion".

To reproduce stuff 1025 open braces or brackets into QMP.

Fix by taking the error exit instead of the normal one.

Reported-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <1448486613-17634-3-git-send-email-armbru@redhat.com>

qobject/json-streamer.c

@@ -68,13 +68,14 @@ static void json_message_process_token(JSONLexer *lexer, QString *token, JSONTok
         /* Security consideration, we limit total memory allocated per object
          * and the maximum recursion depth that a message can force.
          */
-        goto out_emit;
+        goto out_emit_bad;
     }
 
     return;
 
 out_emit_bad:
-    /* clear out token list and tell the parser to emit and error
+    /*
+     * Clear out token list and tell the parser to emit an error
      * indication by passing it a NULL list
      */
     QDECREF(parser->tokens);