Undefined index can occur when data source does not have an snmp_index

[cigamit]

Describe the bug
Under certain circumstances, when rendering a graph, you get an undefined index snmp_query_id in lib/rrd.php on line 2424.

To Reproduce
Steps to reproduce the behavior:

1. Create a graph that uses a replacement value for an option such as the vertical label
2. Attempt to render that graph
3. Goto the Cacti log
4. See error

Expected behavior
No error is generated