logging,cdr,cel: Fix stringfield memory leak.

The stringfields refactor to allow adding stringfields to the end of a
structure (f6f4cf45) exposed some
incomplete cleanup code by some stringfield users.

The most noticeable leaker is the logging system where there is a leak for
every log message generated.

ASTERISK-26078 #close
Reported by:  Etienne Lessard
Patches:
      jira_asterisk_26078_v13.patch (license #5621) patch uploaded
      by Richard Mudgett

Change-Id: If6a08b31336b492c3de6f9dfd07c447f8d5a8782