config: Fix inf loop using ast_category_browse and ast_variable_retrieve

Fix infinite loop when calling ast_variable_retrieve inside an
ast_category_browse loop when there is more than 1 category with
the same name.

Tested-by: George Joseph

Review: https://reviewboard.asterisk.org/r/4089/
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Merged revisions 425713 from http://svn.asterisk.org/svn/asterisk/branches/12
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Merged revisions 425714 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@425715 65c4cc65-6c06-0410-ace0-fbb531ad65f3