Fix column duplication bug in module reload for cdr_pgsql.

Prior to this patch, attempts to reload cdr_pgsql.so would cause the column list to keep
its current data and then add a second copy during the reload. This would cause attempts
to log the CDR to the database to fail. This patch also cleans up some unnecessary null
checks for ast_free and deals with a few potential locking problems.

(closes issue ASTERISK-19216)
Reported by: Jacek Konieczny
Review: https://reviewboard.asterisk.org/r/1711/
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Merged revisions 354263 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 354270 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@354275 65c4cc65-6c06-0410-ace0-fbb531ad65f3