fstrace: Avoid accessing icl log after zapping it

The for loop in icl_EnumerateLogs looks up the next pointer in the
current entry after zapping it. Depending on reference counts, this
may result in us looking up freed memory.

Take a copy of the next point before zapping the current entry, just
in case.

Caught by clang-analyzer

Change-Id: If38f0af2b01c5b8ea00e68e4432c6ad5517578c8
Reviewed-on: http://gerrit.openafs.org/9190
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>