Fix unintential memory retention in stringfields.

* Fix missing / unreachable calls to __ast_string_field_release_active.
* Reset pool->used to zero when the current pool->active reaches zero.

ASTERISK-24307 #close
Reported by: Etienne Lessard
Tested by: ibercom, Etienne Lessard
Review: https://reviewboard.asterisk.org/r/4114/
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Merged revisions 427380 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/11@427381 f38db490-d61c-443f-a65b-d21fe96a405b