config: Fix SEGV in unit test with MALLOC_DEBUG

With MALLOC_DEBUG the /main/config config_basic_ops test was causing a
SEGV while doing an ast_category_delete in an ast_category_browse loop.
Apparently this never worked but was also never tested.  I removed the
test, added 2 notes to config.h indicating that it's not supported and
added a few lines of code to ast_category_delete to prevent the SEGV
should someone attempt it in the future.

Tested-by: George Joseph

Review: https://reviewboard.asterisk.org/r/4078/
........

Merged revisions 425525 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/13@425526 65c4cc65-6c06-0410-ace0-fbb531ad65f3

include/asterisk/config.h

@@ -257,6 +257,10 @@ char *ast_category_browse(struct ast_config *config, const char *prev_name);
  *
  * \retval ast_variable list on success
  * \retval NULL on failure
+ *
+ * \note ast_category_browse is not thread safe and it is not safe to call
+ * ast_category_delete while browsing using ast_category_browse.
+ * ast_category_browse_filtered does not have these restrictions.
  */
 struct ast_variable *ast_variable_browse_filtered(const struct ast_config *config,
 	const char *category_name, const char *filter);
@@ -794,6 +798,9 @@ int ast_category_insert(struct ast_config *config, struct ast_category *cat, con
  * \param category category to delete
  *
  * \return the category after the deleted one which could be NULL.
+ *
+ * \note It is not safe to call ast_category_delete while browsing with
+ * ast_category_browse.  It is safe with ast_category_browse_filtered.
  */
 struct ast_category *ast_category_delete(struct ast_config *cfg, struct ast_category *category);