sysutils/dmidecode: Fix segmentation fault in dmi_hp_240_attr()

Back port a patch from the dmidecode upsteam that resolves a crash when
reading the Attribute #240 on some HP hardware.

See
https://git.savannah.nongnu.org/cgit/dmidecode.git/commit/?id=c3357b532941a8df387618e692e522cc7a43b3e8

Approved by: portmgr blanket
Sponsored by: Klara Inc.