snmp plugin: Check for SNMP_ENDOFMIBVIEW.

snmp plugin: Check for SNMP_ENDOFMIBVIEW.

Some implementations of SNMP will send a special "variable" when you're
leaving the requested subtree with type "endOfMibView" aka.
"SNMP_ENDOFMIBVIEW". Examples of such implementations are Citrix
Netscalers and IBM's General Parallel File System (GPFS). Reading past
this special variable will result in weird values being returned.

This adds an appropriate check to csnmp_check_res_left_subtree().

Thanks to "Mark" for reporting this issue and clarifying.