sysutils/snmp_exporter: Fix snmp_exporter_generator crash

Backport upstream PR #1550 to fix a crash when using net-snmp 5.9.5.0
or later.

PR: 292628
MFH: 2026Q1
Reviewed by: arrowd, jrm
Approved by: arrowd (ports)
Obtained from: https://github.com/prometheus/snmp_exporter/pull/1550
Sponsored by: https://www.patreon.com/bsdivy
Differential Revision: https://reviews.freebsd.org/D55620

(cherry picked from commit 50b9359327b89a6c258f38e336ef5aad4d80eebc)