indexing. When a device has gone it is not removed from device_map
table but just its entry_p field is set to NULL.
So when traversing device_map in disk_OS_get_ATA_disks() and
disk_OS_get_MD_disks() check for entry_p being NULL, otherwise the
bsnmpd crash is possible when a removed map entry is dereferenced.
Before the fix, for disk_OS_get_ATA_disks() the crash could be easily
reproduced running:
atacontrol detach ata1
The crash was not observed in disk_OS_get_MD_disks() because currently
snmp_hostres does no see md(4) disks: to get the device list it uses
devinfo(3), which does not return md devices.
Reported by: Miroslav Lachman 000.fbsd quip.cz
MFC after: 1 week
16c3b091ae