ast_str: Fix improper member access to struct ast_str members.

Accessing members of struct ast_str outside of the string manipulation API
routines is invalid since struct ast_str is supposed to be treated as
opaque.

Review: https://reviewboard.asterisk.org/r/4194/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/11@428244 f38db490-d61c-443f-a65b-d21fe96a405b