In no particular order, changed/fixed all of the following in

ares_parse_txt_reply() current version:

- Fixed a couple of potential double free's.

- Fixed memory leaks upon out of memory condition.

- Fixed pointer arithmetic.

- Setting ntxtreply to zero upon entry for all failure cases.

- Changed data type to size_t for variables substr_len, str_len and
  the length member of ares_txt_reply struct.

- Avoided a couple of memcpy() calls.

- Changed i data type to unsigned int to prevent compiler warnings.

- Adjusted a comment.

- Use ARES_SUCCESS literal for successfull completion.

- Added CVS Id tag.