Consistent memory allocation by ast_bt_get_symbols.

Always use ast_alloc/ast_free. This is handled differently in trunk (r391012).

Review: https://reviewboard.asterisk.org/r/2580/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@396427 f38db490-d61c-443f-a65b-d21fe96a405b