[2.1.8] FreeBSD: Fix potential boot panic with bad label

[ghost]

vdev_geom_read_pool_label() can leave NULL in configs. Check for it and skip consistently when generating rootconf.

Reviewed-by: Alexander Motin mav@FreeBSD.org
Signed-off-by: Ryan Moeller ryan@iXsystems.com
Closes #14291
(cherry picked from commit dc8c2f6)

Motivation and Context

Description

How Has This Been Tested?

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.