MFC r240595:

MFC r240595:

In snmp_hostres, device_map table is used for consistent device table
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

git-svn-id: svn://svn.freebsd.org/base/stable/8@241004 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f