[2.1] zed: Fix zed ASSERT on slot power cycle

[behlendorf]

Motivation and Context

Trivial ZED backport.

Description

We would see zed assert on one of our systems if we powered off a slot. Further examination showed zfs_retire_recv() was reporting a GUID of 0, which in turn would return a NULL nvlist. Add in a check for a zero GUID.

How Has This Been Tested?

Clean backport from master.

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.