nagios_report(): Fix exit code for empty statuses.

When the statuses parameter to the nagios_report() function in
trh_nagioslib is empty, we try to return UNKNOWN as the exit code.
However, we apparently had a brainfart and added one to the value
the .index() method gave us searching the NAGIOSCODES variable.
That's just incorrect; the position of the names in NAGIOSCODES
*is* the right exit code, no offset needed.  Fix that.