config: bug: Fix SEGV in ast_category_insert when matching category isn't found

If you call ast_category_insert with a match category that doesn't exist, the
list traverse runs out of 'next' categories and you get a SEGV.  This patch
adds check for the end-of-list condition and changes the signature to return
an int for success/failure indication instead of a void.

The only consumer of this function is manager and it was also changed to use
the return value.

Tested by: George Joseph
Review: https://reviewboard.asterisk.org/r/3993/
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Merged revisions 423276 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 423277 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 423278 from http://svn.asterisk.org/svn/asterisk/branches/12
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Merged revisions 423279 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@423280 65c4cc65-6c06-0410-ace0-fbb531ad65f3