Undefined variable warnings when updating RRD data

[netniV]

Describe the bug

2019/02/19 13:59:22 - CMDPHP PHP ERROR NOTICE Backtrace: (/plugins/thold/thold.php[135]:thold_edit(), /plugins/thold/thold.php[1195]:get_current_value(), /plugins/thold/thold_functions.php[3623]:rrdtool_function_fetch(), /lib/rrd.php[823]:CactiErrorHandler())
2019/02/19 13:59:22 - ERROR PHP NOTICE: Undefined variable: graph_end in file: /usr/share/cacti/site/lib/rrd.php on line: 823
2019/02/19 13:59:22 - CMDPHP PHP ERROR NOTICE Backtrace: (/plugins/thold/thold.php[135]:thold_edit(), /plugins/thold/thold.php[1195]:get_current_value(), /plugins/thold/thold_functions.php[3623]:rrdtool_function_fetch(), /lib/rrd.php[823]:CactiErrorHandler())
2019/02/19 13:59:22 - ERROR PHP NOTICE: Undefined variable: graph_start in file: /usr/share/cacti/site/lib/rrd.php on line: 823

Additional context
This is caused when thold attempts to retrieve value information. The correct parameters should have been $start_time and $end_time.

[netniV]

This has now been committed to the development code.