res_config_pgsql: Fix a memory leak and use RAII_VAR for cleanup when practical

Review: https://reviewboard.asterisk.org/r/3141/
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Merged revisions 406360 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@406361 65c4cc65-6c06-0410-ace0-fbb531ad65f3