main/config.c: AMI action UpdateConfig EmptyCat clears all categories

When invoking UpdateConfig AMI action with Action set to EmptyCat, Asterisk
will make all categories empty in the config but the one requested with a
Cat variable. This is due to a bug in ast_category_empty (main/config.c)
that makes an incorrect comparison for a category name.

This patch corrects the comparison such that only the requested category
is cleared.

Review: https://reviewboard.asterisk.org/r/3573/

#ASTERISK-23803 #close
Reported by: zvision
patches:
  manager.c.diff uploaded by zvision (License 5755)
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Merged revisions 414880 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 414881 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@414882 f38db490-d61c-443f-a65b-d21fe96a405b