stringfields: Fix bug in ast_string_fields_copy.

ast_string_fields_copy relies on the fact that
__ast_string_field_release_active never previously
zeroed pool->used, so keeping the existing pointer
was "ok".  Now that existing pools can be reset to
'empty', it is important to set each field to
__ast_string_field_empty after releasing the memory.

ASTERISK-24535 #close
Reported by: Corey Farrell
Review: https://reviewboard.asterisk.org/r/4186/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@428272 f38db490-d61c-443f-a65b-d21fe96a405b